plc programming examples.pdf

INST 231 (PLC Programming), section 1 Lab PLC-based motor control system: Question 91 and 92, completed objectives due by the end of day 2, section 2 Exam Day 3 of next section – only a simple calculator may be used! Specific objectives for the “mastery” exam: • Program a start/stop function in a PLC and wire it to control an electromechanical relay (question 93) • Sketch proper wire connections for sourcing or sinking PLC I/O points • Determine status of PLC discrete output given discrete input states and a simple RLL program listing • Calculate either the full-load current or the horsepower of an electric motor (either single- or three-phase) given the line voltage and one of the other parameters • Solve for a specified variable in an algebraic formula • Determine the possibility of suggested faults in a simple PLC circuit given a wiring diagram, RLL program listing, and reported symptoms • INST240 Review: Calculate ranges for hydrostatic (DP) level-measuring instruments given physical dimensions and fluid densities • INST250 Review: Convert between different pressure units (PSI, ”W.C., bar, etc.) • INST262 Review: Identify specific instrument calibration errors (zero, span, linearity, hysteresis) from data in an “As-Found” table Recommended daily schedule Day 1 Theory session topic: Introduction to PLCs Questions 1 through 20; answer questions 1-10 in preparation for discussion (remainder for practice) Day 2 Theory session topic: Contact and coil programming Questions 21 through 40; answer questions 21-30 in preparation for discussion (remainder for practice) Day 3 Theory session topic: Counter instructions Questions 41 through 60; answer questions 41-53 in preparation for discussion (remainder for practice) Day 4 Theory session topic: Timer instructions Questions 61 through 80; answer questions 61-70 in preparation for discussion (remainder for practice) Feedback questions (81 through 90) are optional and may be submitted for review at the end of the day

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Course Syllabus INSTRUCTOR CONTACT INFORMATION: Tony Kuphaldt (360)-752-8477 [office phone] (360)-752-7277 [fax] [email protected] DEPT/COURSE #: INST 231 CREDITS: 3

Lecture Hours: 10

Lab Hours: 50

Work-based Hours: 0

COURSE TITLE: PLC Programming COURSE DESCRIPTION: In this course you will learn how to wire, program, and configure programmable logic controllers (PLCs) to perform discrete control functions including combinational logic, counters, timers, and sequencers. Pre/Corequisite course: INST 230 (Motor Controls) Prerequisite course: MATH&141 (Precalculus 1) COURSE OUTCOMES: Construct, program, and efficiently diagnose control systems incorporating programmable logic controllers (PLCs). COURSE OUTCOME ASSESSMENT: PLC wiring, programming, and configuration outcomes are ensured by measuring student performance against mastery standards, as documented in the Student Performance Objectives. Failure to meet all mastery standards by the next scheduled exam day will result in a failing grade for the course.

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STUDENT PERFORMANCE OBJECTIVES: • Without references or notes, within a limited time (3 hours total for each exam session), independently perform the following tasks. Multiple re-tries are allowed on mastery (100% accuracy) objectives, each with a different set of problems: → Program and connect a PLC to control an electromagnetic relay with 100% accuracy (mastery) → Sketch proper wire connections for sourcing or sinking PLC I/O points given schematic or pictorial diagrams of the components, with 100% accuracy (mastery) → Determine status of a PLC discrete output given input states and a simple RLL program, with 100% accuracy (mastery) → Calculate either the full-load current or the horsepower of an electric motor (either single- or threephase) given the line voltage and one of the other parameters → Solve for specified variables in algebraic formulae, with 100% accuracy (mastery) → Determine the possibility of suggested faults in a simple PLC circuit given measured values (voltage, current), a schematic diagram, and reported symptoms, with 100% accuracy (mastery) → Program a PLC to fulfill a specified control system function • In a team environment and with full access to references, notes, and instructor assistance, perform the following tasks: → Demonstrate proper use of safety equipment and application of safe procedures while using power tools, and working on live systems → Communicate effectively with teammates to plan work, arrange for absences, and share responsibilities in completing all labwork → Construct and commission a motor start/stop system using a PLC as the control element → Generate an accurate wiring diagram compliant with industry standards documenting your team’s motor control system • Independently perform the following tasks on a functioning PLC motor control system with 100% accuracy (mastery). Multiple re-tries are allowed with different specifications/conditions each time): → Diagnose a random fault placed in another team’s PLC motor control system by the instructor within a limited time using no test equipment except a multimeter and ladder logic editing software, logically justifying your steps in the instructor’s direct presence COURSE OUTLINE: A course calendar in electronic format (Excel spreadsheet) resides on the Y: network drive, and also in printed paper format in classroom DMC130, for convenient student access. This calendar is updated to reflect schedule changes resulting from employer recruiting visits, interviews, and other impromptu events. Course worksheets provide comprehensive lists of all course assignments and activities, with the first page outlining the schedule and sequencing of topics and assignment due dates. These worksheets are available in PDF format at http://openbookproject.net/books/socratic/sinst • INST231 Section 1 (PLC contact, coil, and counter programming): 4 days theory and labwork • INST231 Section 2 (PLC timer and sequence programming): 4 days theory and labwork + 1 day for mastery/proportional Exams

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METHODS OF INSTRUCTION: Course structure and methods are intentionally designed to develop critical-thinking and life-long learning abilities, continually placing the student in an active rather than a passive role. • Independent study: daily worksheet questions specify reading assignments, problems to solve, and experiments to perform in preparation (before) classroom theory sessions. Open-note quizzes and work inspections ensure accountability for this essential preparatory work. The purpose of this is to convey information and basic concepts, so valuable class time isn’t wasted transmitting bare facts, and also to foster the independent research ability necessary for self-directed learning in your career. • Classroom sessions: a combination of Socratic discussion, short lectures, small-group problem-solving, and hands-on demonstrations/experiments review and illuminate concepts covered in the preparatory questions. The purpose of this is to develop problem-solving skills, strengthen conceptual understanding, and practice both quantitative and qualitative analysis techniques. • Hands-on PLC programming challenges: daily worksheet questions specify realistic scenarios requiring students to develop real PLC programs on their PLC trainers to implement the desired control function(s). • Lab activities: an emphasis on constructing and documenting working projects (real instrumentation and control systems) to illuminate theoretical knowledge with practical contexts. Special projects off-campus or in different areas of campus (e.g. BTC’s Fish Hatchery) are encouraged. Hands-on troubleshooting exercises build diagnostic skills. • Feedback questions: sets of practice problems at the end of each course section challenge your knowledge and problem-solving ability in current as as well as first year (Electronics) subjects. These are optional assignments, counting neither for nor against your grade. Their purpose is to provide you and your instructor with direct feedback on what you have learned. STUDENT ASSIGNMENTS/REQUIREMENTS: All assignments for this course are thoroughly documented in the following course worksheets located at: http://openbookproject.net/books/socratic/sinst/index.html • INST231 sec1.pdf • INST231 sec2.pdf

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EVALUATION AND GRADING STANDARDS: (out of 100% for the course grade) • Mastery exam and mastery lab objectives = 50% of course grade • Proportional exam = 40% • Lab questions = 10% • Quiz penalty = -1% per failed quiz • Tardiness penalty = -1% per incident (1 “free” tardy per course) • Attendance penalty = -1% per hour (12 hours “sick time” per quarter) • Extra credit = +5% per project All grades are criterion-referenced (i.e. no grading on a “curve”) 100% ≥ A ≥ 95% 90% > B+ ≥ 86% 80% > C+ ≥ 76% 70% > D+ ≥ 66%

95% > A- ≥ 90% 86% > B ≥ 83% 76% > C ≥ 73% 66% > D ≥ 63%

83% > B- ≥ 80% 73% > C- ≥ 70% (minimum passing course grade) 63% > D- ≥ 60% 60% > F

A graded “preparatory” quiz at the start of each classroom session gauges your independent learning prior to the session. A graded “summary” quiz at the conclusion of each classroom session gauges your comprehension of important concepts covered during that session. If absent during part or all of a classroom session, you may receive credit by passing comparable quizzes afterward or by having your preparatory work (reading outlines, work done answering questions) thoroughly reviewed prior to the absence. Absence on a scheduled exam day will result in a 0% score for the proportional exam unless you provide documented evidence of an unavoidable emergency. If you fail a mastery exam, you must re-take a different version of that mastery exam on a different day. Multiple re-tries are allowed, on a different version of the exam each re-try. There is no penalty levied on your course grade for re-taking mastery exams, but failure to successfully pass a mastery exam by the due date (i.e. by the date of the next exam in the course sequence) will result in a failing grade (F) for the course. If any other “mastery” objectives are not completed by their specified deadlines, your overall grade for the course will be capped at 70% (C- grade), and you will have one more school day to complete the unfinished objectives. Failure to complete those mastery objectives by the end of that extra day (except in the case of documented, unavoidable emergencies) will result in a failing grade (F) for the course. “Lab questions” are assessed by individual questioning, at any date after the respective lab objective (mastery) has been completed by your team. These questions serve to guide your completion of each lab exercise and confirm participation of each individual student. Grading is as follows: full credit for thorough, correct answers; half credit for partially correct answers; and zero credit for major conceptual errors. All lab questions must be answered by the due date of the lab exercise. Extra credit opportunities exist for each course, and may be assigned to students upon request. The student and the instructor will first review the student’s performance on feedback questions, homework, exams, and any other relevant indicators in order to identify areas of conceptual or practical weakness. Then, both will work together to select an appropriate extra credit activity focusing on those identified weaknesses, for the purpose of strengthening the student’s competence. A due date will be assigned (typically two weeks following the request), which must be honored in order for any credit to be earned from the activity. Extra credit may be denied at the instructor’s discretion if the student has not invested the necessary preparatory effort to perform well (e.g. lack of preparation for daily class sessions, poor attendance, no feedback questions submitted, etc.).

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REQUIRED STUDENT SUPPLIES AND MATERIALS: • Course worksheets available for download in PDF format • Lessons in Industrial Instrumentation textbook, available for download in PDF format → Access worksheets and book at: http://openbookproject.net/books/socratic/sinst • Spiral-bound notebook for reading annotation, homework documentation, and note-taking. • Instrumentation reference CD-ROM (free, from instructor). This disk contains many tutorials and datasheets in PDF format to supplement your textbook(s). • Tool kit (see detailed list) • Simple scientific calculator (non-programmable, non-graphing, no unit conversions, no numeration system conversions), TI-30Xa or TI-30XIIS recommended • Small “brick” PLC and HMI panel (Automation Direct option): → Automation Direct CLICK PLC model C0-00DD1-D (price ≈ $70) 8 discrete (DC) inputs, 6 discrete (DC) outputs → or Automation Direct CLICK PLC model C0-02DD1-D (price ≈ $130) 4 discrete (DC) inputs, 4 discrete (DC) outputs, 2 analog inputs, 2 analog outputs, RS-485 Modbus communications port, real-time clock and calendar → Automation Direct CLICK 24 VDC power supply model C0-00AC (price ≈ $30) 24 VDC at 0.5 amp maximum output → Automation Direct C-More Micro HMI panel 3 inch EA1-S3ML-N (price ≈ $150) → optional Automation Direct C-More Micro touch-screen HMI panel 3 inch EA1-S3ML (price ≈ $190) → Automation Direct USB/serial adapter and cable part EA-MG-PGM-CBL (price ≈ $40) necessary for programming the C-More Micro HMI panel (also works for programming the PLC) → Note: We have found the Autmoation Direct software works equally well through a 9-pin serial port as through a USB port (with converter), and is very “friendly” to use. • Small “brick” PLC and HMI panel (Allen-Bradley option): → Rockwell (Allen-Bradley) MicroLogix 1000 model 1761-L10BWA (price ≈ $85 with BTC student discount at North Coast Electric) 6 discrete (DC) inputs, 4 discrete (relay) outputs → or Rockwell (Allen-Bradley) MicroLogix 1100 model 1763-L16BWA (price ≈ $240 with BTC student discount at North Coast Electric) 10 discrete (DC) inputs, 6 discrete (DC) outputs, 2 analog inputs, RS-485 communication port, 10 Mbit/s Ethernet communication port, embedded web server for remote monitoring of data points (series A or B programmable using free MicroLogix Lite software) → Rockwell (Allen-Bradley) cable part 1761-CBL-PM02 (price ≈ $30 with BTC student discount at North Coast Electric) → Automation Direct C-More Micro HMI panel 3 inch EA1-S3ML-N (price ≈ $150) → optional Automation Direct C-More Micro touch-screen HMI panel 3 inch EA1-S3ML (price ≈ $190) → Automation Direct cable part EA-MLOGIX-CBL (price ≈ $30) and adapter part EA-MG-SP1 (price ≈ $50) necessary for connecting the C-More Micro HMI panel to an Allen-Bradley MicroLogix 1000 PLC → Automation Direct USB/serial adapter and cable part EA-MG-PGM-CBL (price ≈ $40) necessary for programming the C-More Micro HMI panel → Note: Programming Allen-Bradley PLCs is best done using a PC with a 9-pin serial port. We have found trying to use a USB-to-serial adapter very troublesome with Allen-Bradley software!

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ADDITIONAL INSTRUCTIONAL RESOURCES: • The BTC Library hosts a substantial collection of textbooks and references on the subject of Instrumentation, as well as links in its online catalog to free Instrumentation e-book resources available on the Internet. • “BTCInstrumentation” channel on YouTube (http://www.youtube.com/BTCInstrumentation), hosts a variety of short video tutorials and demonstrations on instrumentation. • ISA Student Section at BTC meets regularly to set up industry tours, raise funds for scholarships, and serve as a general resource for Instrumentation students. Membership in the ISA is $10 per year, payable to the national ISA organization. Membership includes a complementary subscription to InTech magazine. • ISA website (http://www.isa.org) provides all of its standards in electronic format, many of which are freely available to ISA members. • Cad Standard (CadStd) or similar AutoCAD-like drafting software (useful for sketching loop and wiring diagrams). Cad Standard is a simplified clone of AutoCAD, and is freely available at: http://www.cadstd.com • To receive classroom accommodations, registration with Disability Support Services (DSS) is required. Call 360-752-8450, email [email protected], or visit the DSS office in the Counseling and Career Center (room 106, College Services building).

file INST231syllabus 7

Sequence of second-year Instrumentation courses

Core Electronics -- 3 qtrs including MATH 141 (Precalculus 1)

(Only if 4th quarter was Summer: INST23x)

INST 200 -- 1 wk Intro. to Instrumentation

Prerequisite for all INST24x, INST25x, and INST26x courses

Summer quarter

Fall quarter

Winter quarter

Offered 1st week of Fall, Winter, and Spring quarters

Spring quarter

INST 230 -- 3 cr

INST 240 -- 6 cr

INST 250 -- 5 cr

INST 260 -- 4 cr

Motor Controls

Pressure/Level Measurement

Final Control Elements

Data Acquisition Systems

INST 231 -- 3 cr

INST 241 -- 6 cr

INST 251 -- 5 cr

INST 262 -- 5 cr

PLC Programming

Temp./Flow Measurement

PID Control

DCS and Fieldbus

INST 232 -- 3 cr

INST 242 -- 5 cr

INST 252 -- 4 cr

INST 263 -- 5 cr

Loop Tuning

Control Strategies

PLC Systems

Analytical Measurement

PTEC 107 -- 5 cr

ENGT 122 -- 6 cr

Process Science

CAD 1: Basics

Prerequisite for INST206

All courses completed?

Yes

INST 205 -- 1 cr Job Prep I No INST 206 -- 1 cr Job Prep II

Graduate!!!

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Offered 1st week of Fall, Winter, and Spring quarters

The particular sequence of courses you take during the second year depends on when you complete all first-year courses and enter the second year. Since students enter the second year of Instrumentation at four different times (beginnings of Summer, Fall, Winter, and Spring quarters), the particular course sequence for any student will likely be different from the course sequence of classmates. Some second-year courses are only offered in particular quarters with those quarters not having to be in sequence, while others are offered three out of the four quarters and must be taken in sequence. The following layout shows four typical course sequences for second-year Instrumentation students, depending on when they first enter the second year of the program:

Possible course schedules depending on date of entry into 2nd year Beginning in Summer July

Summer quarter

Beginning in Fall Sept.

Sept.

Motor Controls

Intro. to Instrumentation



INST 231 -- 3 cr

INST 240 -- 6 cr

INST 250 -- 5 cr

INST 260 -- 4 cr

PLC Programming




PLC Systems

Fall quarter

Dec. Jan.

INST 251 -- 5 cr

INST 262 -- 5 cr

PID Control

DCS and Fieldbus

INST 242 -- 5 cr

INST 252 -- 4 cr

INST 263 -- 5 cr

Loop Tuning

Control Strategies


INST 241 -- 6 cr

INST 250 -- 5 cr



INST 242 -- 5 cr Analytical Measurement

Winter quarter


Mar. April

INST 251 -- 5 cr

PTEC 107 -- 5 cr

Winter quarter


INST 250 -- 5 cr

Mar. April

Process Science

Spring quarter

ENGT 122 -- 6 cr June July

CAD 1: Basics

Summer quarter

INST 205 -- 1 cr Job Prep I

INST 230 -- 3 cr

INST 251 -- 5 cr

INST 260 -- 4 cr

INST 231 -- 3 cr

PID Control


PLC Programming

INST 252 -- 4 cr

INST 262 -- 5 cr

Loop Tuning

DCS and Fieldbus

PTEC 107 -- 5 cr

INST 263 -- 5 cr

Process Science

Control Strategies

Motor Controls

INST 232 -- 3 cr Aug. Sept.

June July

PLC Systems

Fall quarter INST 205 -- 1 cr Job Prep I

ENGT 122 -- 6 cr

Spring quarter INST 206 -- 1 cr Job Prep II

PID Control

CAD 1: Basics

INST 240 -- 6 cr Pressure/Level Measurement

Summer quarter

INST 252 -- 4 cr

INST 260 -- 4 cr

INST 230 -- 3 cr

INST 241 -- 6 cr

Loop Tuning


Motor Controls


PTEC 107 -- 5 cr

INST 262 -- 5 cr

INST 231 -- 3 cr

Process Science

DCS and Fieldbus

PLC Programming

INST 260 -- 4 cr Data Acquisition Systems

INST 232 -- 3 cr

INST 263 -- 5 cr

Spring quarter

Control Strategies

INST 206 -- 1 cr Job Prep II

Aug.

ENGT 122 -- 6 cr June July

Sept.

CAD 1: Basics

Summer quarter

PLC Systems

INST 242 -- 5 cr Dec. Jan.


Winter quarter INST 206 -- 1 cr Job Prep II

Fall quarter INST 206 -- 1 cr Job Prep II

INST 250 -- 5 cr Final Control Elements

INST 262 -- 5 cr

INST 230 -- 3 cr

INST 240 -- 6 cr

INST 251 -- 5 cr

DCS and Fieldbus

Motor Controls


PID Control

INST 263 -- 5 cr

INST 231 -- 3 cr

INST 241 -- 6 cr

INST 252 -- 4 cr

Control Strategies

PLC Programming


Loop Tuning

INST 232 -- 3 cr

ENGT 122 -- 6 cr June

INST 241 -- 6 cr Temp./Flow Measurement



April

Spring quarter INST 200 -- 1 wk

INST 240 -- 6 cr

Mar.

April

INST 200 -- 1 wk


Jan.

Winter quarter

INST 200 -- 1 wk

INST 200 -- 1 wk

Dec.

Jan.

Fall quarter

Beginning in Spring

INST 230 -- 3 cr

INST 232 -- 3 cr Aug.

Beginning in Winter

CAD 1: Basics

Graduation!

Aug.

INST 242 -- 5 cr

PLC Systems

Dec.

Graduation!


Graduation!

file sequence 9

PTEC 107 -- 5 cr Mar.

Process Science

Graduation!

General student expectations Your future employer expects you to: show up for work on time, prepared, every day; to work safely, efficiently, conscientiously, and with a clear mind; to be self-directed and take initiative; to follow through on all commitments; and to take responsibility for all your actions and for the consequences of those actions. Instrument technicians work on highly complex, mission-critical measurement and control systems, where incompetence and/or lack of integrity invites disaster. This is also why employers check legal records and social networking websites for signs of irresponsibility when considering a graduate for hire. Substance abuse is particularly noteworthy since it impairs reasoning, and this is first and foremost a “thinking” career. Mastery You are expected to master the fundamentals of your chosen craft. Accordingly, you will be challenged with “mastery” objectives ensuring 100% competence in specific knowledge and skill areas (with multiple opportunities to re-try if necessary). Failure to fulfill any mastery objective(s) by the deadline results in your grade for that course being capped at a C-, with one more day given to demonstrate mastery. Failure to fulfill any mastery objective(s) by the end of that extra day results in a failing grade for the course. Punctuality and Attendance You are expected to arrive on time, every scheduled day, and attend all day, just as you would for a job. If a session begins at 12:00 noon, 12:00:01 is considered late. Each student has 12 “sick hours” per quarter applicable to absences not verifiably employment-related, school-related, weather-related, or required by law. Each student must confer with the instructor to apply “sick hours” to any missed time – this is not done automatically for the student. Students may donate unused “sick hours” to whomever they specifically choose. You must contact your instructor and team members immediately if you know you will be late or absent, and it is your responsibility to catch up on all missed activities. Absence on an exam day will result in a zero score for that exam, unless due to a documented emergency. Independent study Industry advisors and successful graduates consistently identify independent learning as the most important skill to possess for this career. You will build this vital skill by working through each day’s assigned reading and homework problems before class begins. You may not be able to answer every question on your own, but you are expected to do your best and to identify as specifically as possible where you experienced trouble. It is your responsibility to check the course schedule (given on the front page of every worksheet) to identify assignments and due dates. Most students find 3 or more hours per day the a typical time commitment for adequate study. Question 0 (included in every worksheet) lists practical tips for independent learning and problem-solving. Safety You are expected to work safely in the lab just as you will be on the job. This includes wearing proper attire (safety glasses and closed-toed shoes in the lab at all times), implementing lock-out/tag-out procedures when working on circuits over 24 volts, using ladders to reach high places rather than standing on tables or chairs, and maintaining an orderly work environment. Teamwork You will work in instructor-assigned teams to complete lab assignments, just as you will work in teams to complete complex assignments on the job. As part of a team, you must keep your teammates informed of your whereabouts in the event you must step away from the lab or cannot attend for any reason. Any student regularly compromising team performance through lack of participation, absence, tardiness, disrespect, unsafe work, or other disruptive behavior(s) will be given the choice of either completing all labwork independently for the remainder of the quarter or receiving a failing grade for the course. Responsibility for actions If you lose or damage college property (e.g. lab equipment), you must find, repair, or help replace it. If your actions strain the relationship between the program and an employer (e.g. poor behavior during a tour or an internship), you must make amends. The general rule here is this: “If you break it, you fix it!” Disciplinary action The Student Code of Conduct (Washington Administrative Codes WAC 495B-120) explicitly authorizes disciplinary action against misconduct including: academic dishonesty (e.g. cheating, plagiarism), dangerous or lewd behavior, theft, harassment, intoxication, destruction of property, or disruption of the learning environment. 10

General student expectations (continued) Formal learning is a partnership between instructor and student: both are responsible for maximizing learning. Your instructors’ responsibilities include – but are not limited to – maintaining an environment conducive to learning, providing necessary learning resources, continuously testing your comprehension, dispensing appropriate advice, and actively challenging you to think deeper than you would be inclined to do on your own (just like an athletic trainer will “push” their clients to go faster, farther, and work harder than they would otherwise do on their own). Your responsibilities as a student include – but are not limited to – prioritizing time for study, utilizing all learning resources offered to you, heeding your instructor’s advice, and above all taking your role as a learner seriously. The single most important factor in any student’s education is that student’s dedication. The most talented instructor, at the most well-equipped institution, is worthless if the student doesn’t care to learn. Conversely, virtually no circumstance can prevent a dedicated student from learning whatever they want. In order to clearly illustrate what dedication to learning looks like from a student’s perspective, the following clarifications are given: You are here to learn, not to receive a high grade, not to earn a degree, and not even to get a job. If you make learning your first priority, you will attain all those other goals as a bonus. If, however, you attempt to achieve those secondary goals to the exclusion of learning, you will seriously compromise your long-term success in this career, and you will have wasted your time here. Memorization alone is not learning. Sadly, many students’ educational experiences lead them to believe learning is nothing more than an accumulation of facts and procedures, when in truth you will need to do much more than memorize information in order to be successful as an instrument technician. True learning is gaining the ability to think in new ways. The “gold standard” of learning is when you have grasped a concept so well that you are able to apply it in creative ways to applications and contexts completely new to you. In fact, this is a simple way for you to test your own learning: see how well you are able to apply it to new scenarios. Observation alone is not learning. Merely watching someone else perform a task, execute a procedure, or solve a problem does not mean you are proficient in the same, any more than watching an athlete play the game means you now can play at the same skill level. Unless and until you can consistently and independently demonstrate competence, you haven’t learned. The goal of any learning activity is to master the underlying principles, not merely to complete the activity. The instructor does not need your answers to homework problems. The instructor does not need your completed lab project. What the instructor needs is a demonstration of your capabilities. The activity itself is nothing more than a means to an end – merely a tool for sharpening skills and demonstrating competence. As such, you should never mistake the result of the activity (a finished product) for the goal of the activity (a new ability). The most important question to ask “Why?” Ask yourself this question constantly as you learn new things. Why does this new concept work the way it does? Why does this procedure produce results? Why are we learning this skill? Why does the instructor keep referring me to the literature instead of just giving me the answer I need? “Why” is a catalyst for deep understanding. There are no shortcuts to learning. Relying on classmates for answers rather than figuring them out for yourself, skipping learning activities because you think they’re too challenging or take too long, and other similar “shortcuts” do nothing to help you learn. Let me be clear on this point: I am not advising you to avoid shortcuts in your learning; I’m telling you shortcuts to learning don’t actually exist at all. Any time you think you’ve discovered a shortcut to learning, what you have actually done is find a way to avoid learning. Acquiring and mastering a new ability is hard work – always! Accept this fact and do the hard work necessary to learn. file expectations 11

General tool and supply list Wrenches • Combination (box- and open-end) wrench set, 1/4” to 3/4” – the most important wrench sizes are 7/16”, 1/2”, 9/16”, and 5/8”; get these immediately! • Adjustable wrench, 6” handle (sometimes called “Crescent” wrench) • Hex wrench (“Allen” wrench) set, fractional – 1/16” to 3/8” • Optional: Hex wrench (“Allen” wrench) set, metric – 1.5 mm to 10 mm • Optional: Miniature combination wrench set, 3/32” to 1/4” (sometimes called an “ignition wrench” set) Note: when turning a bolt, nut, or tube fitting with a hexagonal body, the preferred ranking of hand tools to use (from first to last) is box-end wrench or socket, open-end wrench, and finally adjustable wrench. Pliers should never be used to turn the head of a fitting or fastener unless it is absolutely unavoidable! Pliers • Needle-nose pliers • Tongue-and-groove pliers (sometimes called “Channel-lock” pliers) • Diagonal wire cutters (sometimes called “dikes”) Screwdrivers • Slotted, 1/8” and 1/4” shaft • Phillips, #1 and #2 • Jeweler’s screwdriver set • Optional: Magnetic multi-bit screwdriver (e.g. Klein Tools model 70035) Measurement tools • Tape measure. 12 feet minimum • Optional: Vernier calipers • Optional: Bubble level Electrical • Multimeter, Fluke model 87-IV or better • Wire strippers/terminal crimpers with a range including 10 AWG to 18 AWG wire • Soldering iron, 10 to 25 watt • Rosin-core solder • Package of compression-style fork terminals (e.g. Thomas & Betts “Sta-Kon” part number 14RB-10F, 14 to 18 AWG wire size, #10 stud size) Safety • Safety glasses or goggles (available at BTC bookstore) • Earplugs (available at BTC bookstore) Miscellaneous • Simple scientific calculator (non-programmable, non-graphing, no unit conversions, no numeration system conversions), TI-30Xa or TI-30XIIS recommended. Required for some exams! • Teflon pipe tape • Utility knife • Optional: Flashlight An inexpensive source of high-quality tools is your local pawn shop. Look for name-brand tools with unlimited lifetime guarantees (e.g. Sears “Craftsman” brand, Snap-On, etc.). Some local tool suppliers give BTC student discounts as well! file tools 12

Methods of instruction This course develops self-instructional and diagnostic skills by placing students in situations where they are required to research and think independently. In all portions of the curriculum, the goal is to avoid a passive learning environment, favoring instead active engagement of the learner through reading, reflection, problem-solving, and experimental activities. The curriculum may be roughly divided into two portions: theory and practical.

Theory In the theory portion of each course, students independently research subjects prior to entering the classroom for discussion. This means working through all the day’s assigned questions as completely as possible. This usually requires a fair amount of technical reading, and may also require setting up and running simple experiments. At the start of the classroom session, the instructor will check each student’s preparation with a quiz. Students then spend the rest of the classroom time working in groups and directly with the instructor to thoroughly answer all questions assigned for that day, articulate problem-solving strategies, and to approach the questions from multiple perspectives. To put it simply: fact-gathering happens outside of class and is the individual responsibility of each student, so that class time may be devoted to the more complex tasks of critical thinking and problem solving where the instructor’s attention is best applied. Classroom theory sessions usually begin with either a brief Q&A discussion or with a “Virtual Troubleshooting” session where the instructor shows one of the day’s diagnostic question diagrams while students propose diagnostic tests and the instructor tells those students what the test results would be given some imagined (“virtual”) fault scenario, writing the test results on the board where all can see. The students then attempt to identify the nature and location of the fault, based on the test results. Each student is free to leave the classroom when they have completely worked through all problems and have answered a “summary” quiz designed to gauge their learning during the theory session. If a student finishes ahead of time, they are free to leave, or may help tutor classmates who need extra help. The express goal of this “inverted classroom” teaching methodology is to help each student cultivate critical-thinking and problem-solving skills, and to sharpen their abilities as independent learners. While this approach may be very new to you, it is more realistic and beneficial to the type of work done in instrumentation, where critical thinking, problem-solving, and independent learning are “must-have” skills.

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Lab In the lab portion of each course, students work in teams to install, configure, document, calibrate, and troubleshoot working instrument loop systems. Each lab exercise focuses on a different type of instrument, with a eight-day period typically allotted for completion. An ordinary lab session might look like this: (1) Start of practical (lab) session: announcements and planning (a) The instructor makes general announcements to all students (b) The instructor works with team to plan that day’s goals, making sure each team member has a clear idea of what they should accomplish (2) Teams work on lab unit completion according to recommended schedule: (First day) Select and bench-test instrument(s) (One day) Connect instrument(s) into a complete loop (One day) Each team member drafts their own loop documentation, inspection done as a team (with instructor) (One or two days) Each team member calibrates/configures the instrument(s) (Remaining days, up to last) Each team member troubleshoots the instrument loop (3) End of practical (lab) session: debriefing where each team reports on their work to the whole class Troubleshooting assessments must meet the following guidelines: • Troubleshooting must be performed on a system the student did not build themselves. This forces students to rely on another team’s documentation rather than their own memory of how the system was built. • Each student must individually demonstrate proper troubleshooting technique. • Simply finding the fault is not good enough. Each student must consistently demonstrate sound reasoning while troubleshooting. • If a student fails to properly diagnose the system fault, they must attempt (as many times as necessary) with different scenarios until they do, reviewing any mistakes with the instructor after each failed attempt.

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Distance delivery methods Sometimes the demands of life prevent students from attending college 6 hours per day. In such cases, there exist alternatives to the normal 8:00 AM to 3:00 PM class/lab schedule, allowing students to complete coursework in non-traditional ways, at a “distance” from the college campus proper. For such “distance” students, the same worksheets, lab activities, exams, and academic standards still apply. Instead of working in small groups and in teams to complete theory and lab sections, though, students participating in an alternative fashion must do all the work themselves. Participation via teleconferencing, video- or audio-recorded small-group sessions, and such is encouraged and supported. There is no recording of hours attended or tardiness for students participating in this manner. The pace of the course is likewise determined by the “distance” student. Experience has shown that it is a benefit for “distance” students to maintain the same pace as their on-campus classmates whenever possible. In lieu of small-group activities and class discussions, comprehension of the theory portion of each course will be ensured by completing and submitting detailed answers for all worksheet questions, not just passing daily quizzes as is the standard for conventional students. The instructor will discuss any incomplete and/or incorrect worksheet answers with the student, and ask that those questions be re-answered by the student to correct any misunderstandings before moving on. Labwork is perhaps the most difficult portion of the curriculum for a “distance” student to complete, since the equipment used in Instrumentation is typically too large and expensive to leave the school lab facility. “Distance” students must find a way to complete the required lab activities, either by arranging time in the school lab facility and/or completing activities on equivalent equipment outside of school (e.g. at their place of employment, if applicable). Labwork completed outside of school must be validated by a supervisor and/or documented via photograph or videorecording. Conventional students may opt to switch to “distance” mode at any time. This has proven to be a benefit to students whose lives are disrupted by catastrophic events. Likewise, “distance” students may switch back to conventional mode if and when their schedules permit. Although the existence of alternative modes of student participation is a great benefit for students with challenging schedules, it requires a greater investment of time and a greater level of self-discipline than the traditional mode where the student attends school for 6 hours every day. No student should consider the “distance” mode of learning a way to have more free time to themselves, because they will actually spend more time engaged in the coursework than if they attend school on a regular schedule. It exists merely for the sake of those who cannot attend during regular school hours, as an alternative to course withdrawal.

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General advice for successful learning Focus on principles, not procedures • Effective problem-solvers don’t bother trying to memorize procedures for problem-solving because procedures are too specific to the type of problem. Rather, they internalize general principles applicable to a wide variety of problems. • When asking questions about some new subject, concentrate on “why” rather than “how” or “what.” Cultivate meta-cognitive skills (the ability to monitor your own thinking on a subject)! • Whenever you get “stuck” trying to understand a concept, clearly identify where you are getting stuck, and where things stop making sense. • When you think you understand a concept, test your understanding by explaining it in your own words. You can do this by trying to explain it to a willing classmate, or by imagining yourself trying to explain it to someone. If you cannot clearly explain a concept to someone else, you do not understand it well enough yourself ! • The technique of trying to explain a concept also works well to identify where you are stuck. The point at which you find yourself unable to clearly articulate the concept is very likely the exact point of your misconception or confusion. Join or create a study group with like-minded classmates! • Read the textbook assignments together. • Solve assigned problems together. • Collectively identify difficult concepts and areas needing clarification, to bring up later during class. • Take turns trying to explain complicated concepts to each other, then critiquing those explanations. Eliminate distractions in your life! • Time-wasting technologies: televisions, internet, video games, mobile phones, etc. • Unhelpful friends, unhealthy relationships, etc. Make use of “wasted” time to study! • Carefully plan your lab sessions with your teammates to reserve a portion of each day’s lab time for study. • Bring a meal to school every day and use your one-hour lunch break for study instead of eating out. This will not just save you time, but also money! • Plan to arrive at school at least a half-hour early (the doors unlock at 7:00 AM) and use the time to study as opposed to studying late at night. This also helps guard against tardiness in the event of unexpected delays, and ensures you a better parking space! Take responsibility for your learning and your life! • Do not procrastinate, waiting until the last minute to do something. • Obtain all the required books, and any supplementary study materials available to you. If the books cost too much, look on the internet for used texts (www.amazon.com, www.half.com, etc.) and use the money from the sale of your television and video games to buy them! • Make an honest attempt to solve problems before asking someone else to help you. Being able to problem-solve is a skill that will improve only if you continue to work at it. • If you detect trouble understanding a basic concept, address it immediately. Never ignore an area of confusion, believing you will pick up on it later. Later may be too late! • Do not wait for others to do things for you. No one is going to make extra effort purely on your behalf. . . . And the number one tip for success . . . • Realize that there are no shortcuts to learning. Every time you seek a shortcut, you are actually cheating yourself out of a learning opportunity!! file studytips 16

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Note: the text on this page is not a license. It is simply a handy reference for understanding the Legal Code (the full license) - it is a human-readable expression of some of its key terms. Think of it as the user-friendly interface to the Legal Code beneath. This simple explanation itself has no legal value, and its contents do not appear in the actual license.

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Metric prefixes and conversion constants • • • • • • • • • • • • • • • • • • • • •

Metric prefixes Yotta = 1024 Symbol: Y Zeta = 1021 Symbol: Z Exa = 1018 Symbol: E Peta = 1015 Symbol: P Tera = 1012 Symbol: T Giga = 109 Symbol: G Mega = 106 Symbol: M Kilo = 103 Symbol: k Hecto = 102 Symbol: h Deca = 101 Symbol: da Deci = 10−1 Symbol: d Centi = 10−2 Symbol: c Milli = 10−3 Symbol: m Micro = 10−6 Symbol: µ Nano = 10−9 Symbol: n Pico = 10−12 Symbol: p Femto = 10−15 Symbol: f Atto = 10−18 Symbol: a Zepto = 10−21 Symbol: z Yocto = 10−24 Symbol: y METRIC PREFIX SCALE T tera 1012

G M giga mega 109 106

k kilo 103

(none) 100

m µ milli micro 10-3 10-6

102 101 10-1 10-2 hecto deca deci centi h da d c

• • • • •

Conversion formulae for temperature o F = (o C)(9/5) + 32 o C = (o F - 32)(5/9) o R = o F + 459.67 K = o C + 273.15 Conversion equivalencies for distance 1 inch (in) = 2.540000 centimeter (cm) 1 foot (ft) = 12 inches (in) 1 yard (yd) = 3 feet (ft) 1 mile (mi) = 5280 feet (ft)

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n nano 10-9

p pico 10-12

Conversion equivalencies for volume 1 gallon (gal) = 231.0 cubic inches (in3 ) = 4 quarts (qt) = 8 pints (pt) = 128 fluid ounces (fl. oz.) = 3.7854 liters (l) 1 milliliter (ml) = 1 cubic centimeter (cm3 )

Conversion equivalencies for velocity 1 mile per hour (mi/h) = 88 feet per minute (ft/m) = 1.46667 feet per second (ft/s) = 1.60934 kilometer per hour (km/h) = 0.44704 meter per second (m/s) = 0.868976 knot (knot – international)

Conversion equivalencies for mass 1 pound (lbm) = 0.45359 kilogram (kg) = 0.031081 slugs

Conversion equivalencies for force 1 pound-force (lbf) = 4.44822 newton (N)

Conversion equivalencies for area 1 acre = 43560 square feet (ft2 ) = 4840 square yards (yd2 ) = 4046.86 square meters (m2 )

Conversion equivalencies for common pressure units (either all gauge or all absolute) 1 pound per square inch (PSI) = 2.03602 inches of mercury (in. Hg) = 27.6799 inches of water (in. W.C.) = 6.894757 kilo-pascals (kPa) = 0.06894757 bar 1 bar = 100 kilo-pascals (kPa) = 14.504 pounds per square inch (PSI)

Conversion equivalencies for absolute pressure units (only) 1 atmosphere (Atm) = 14.7 pounds per square inch absolute (PSIA) = 101.325 kilo-pascals absolute (kPaA) = 1.01325 bar (bar) = 760 millimeters of mercury absolute (mmHgA) = 760 torr (torr)

Conversion equivalencies for energy or work 1 british thermal unit (Btu – “International Table”) = 251.996 calories (cal – “International Table”) = 1055.06 joules (J) = 1055.06 watt-seconds (W-s) = 0.293071 watt-hour (W-hr) = 1.05506 x 1010 ergs (erg) = 778.169 foot-pound-force (ft-lbf)

Conversion equivalencies for power 1 horsepower (hp – 550 ft-lbf/s) = 745.7 watts (W) = 2544.43 british thermal units per hour (Btu/hr) = 0.0760181 boiler horsepower (hp – boiler)

Acceleration of gravity (free fall), Earth standard 9.806650 meters per second per second (m/s2 ) = 32.1740 feet per second per second (ft/s2 )

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Physical constants Speed of light in a vacuum (c) = 2.9979 × 108 meters per second (m/s) = 186,281 miles per second (mi/s) Avogadro’s number (NA ) = 6.022 × 1023 per mole (mol−1 ) Electronic charge (e) = 1.602 × 10−19 Coulomb (C) Boltzmann’s constant (k) = 1.38 × 10−23 Joules per Kelvin (J/K) Stefan-Boltzmann constant (σ) = 5.67 × 10−8 Watts per square meter-Kelvin4 (W/m2 ·K4 ) Molar gas constant (R) = 8.314 Joules per mole-Kelvin (J/mol-K) Properties of Water Freezing point at sea level = 32o F = 0o C Boiling point at sea level = 212o F = 100o C Density of water at 4o C = 1000 kg/m3 = 1 g/cm3 = 1 kg/liter = 62.428 lb/ft3 = 1.94 slugs/ft3 Specific heat of water at 14o C = 1.00002 calories/g·o C = 1 BTU/lb·o F = 4.1869 Joules/g·o C Specific heat of ice ≈ 0.5 calories/g·o C Specific heat of steam ≈ 0.48 calories/g·o C Absolute viscosity of water at 20o C = 1.0019 centipoise (cp) = 0.0010019 Pascal-seconds (Pa·s) Surface tension of water (in contact with air) at 18o C = 73.05 dynes/cm pH of pure water at 25o C = 7.0 (pH scale = 0 to 14) Properties of Dry Air at sea level Density of dry air at 20o C and 760 torr = 1.204 mg/cm3 = 1.204 kg/m3 = 0.075 lb/ft3 = 0.00235 slugs/ft3 Absolute viscosity of dry air at 20o C and 760 torr = 0.018 centipoise (cp) = 1.8 × 10−5 Pascalseconds (Pa·s)

file conversion constants 20

Question 0 How to read actively: • Articulate your thoughts as you read. This will develop metacognition, which is the supervision of your own thoughts. You should note any interesting words and patterns used by the author, pose questions as they occur to you, state when and where you get confused by the text, clarify where the author cites facts versus makes a judgment or states an opinion, identify cross-references with illustrations and other passages of text, list common themes and principles, etc. • Make the ideas your own by summarizing everything you read. This is far more effective than shallow annotation methods such as underlining and highlighting. A suggested ratio is writing or speaking one sentence of your own thoughts per paragraph of text read. • Work through all mathematical exercises used within the text to explain concepts. Although it may seem pointless to do what the author has already done you, this will help you identify potential misunderstandings that might otherwise go unnoticed. • Maintain a notebook documenting general principles and important formulae you encounter. • Imagine trying to explain what you’ve learned to an intelligent child at the end of each learning section (e.g. before the exam). Teaching forces you to distill concepts to their essence, and by doing so helps you clarify those concepts and expose misconceptions. Your first attempt won’t be perfect, but subsequent attempts will get better and better. Once you have a satisfactory explanation, express it in the fewest words possible without oversimplification. Follow Albert Einstein’s advice here: “Everything should be made as simple as possible, but no simpler.” Problem-solving tips: • Apply active reading strategies to any written problems so you know exactly what’s being asked of you to solve. • Identify all general principles applicable to the problem, then identify how the goal of the problem (i.e. what it is you’re asked to solve) and the “given” information fits with those principles. • Sketch a diagram to organize all “given” information and show where the answer will fit. • Perform “thought experiments” to visualize the effects of different conditions. • Simplify the problem and then solve that simplified problem to identify strategies applicable to the original problem (e.g. change quantitative to qualitative, or visa-versa; substitute different numerical values to make them easier to work with; eliminate confusing details; add details to eliminate unknowns; consider limiting cases that are easier to grasp; put the problem into a more familiar context, or analogy). • Work “backward” from a hypothetical solution to a new set of given conditions. Above all, cultivate persistence in your studies. Persistent effort is necessary to master anything non-trivial. The keys to persistence are (1) having the desire to achieve that mastery, and (2) knowing that challenges are normal and not an indication of something gone wrong. A common error is to equate easy with effective: students often believe learning should be easy if everything is done right. The truth is that mastery never comes easy, and that “easier” methods usually substitute memorization for understanding!

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Questions Question 1 Read and outline the introduction and “PLC Examples” section of the “Programmable Logic Controllers” chapter in your Lessons In Industrial Instrumentation textbook. Note the page numbers where important illustrations, photographs, equations, tables, and other relevant details are found. Prepare to thoughtfully discuss with your instructor and classmates the concepts and examples explored in this reading. file i00460 Question 2 Read selected portions of the Siemens “SIMATIC S7-200 Programmable Controller System Manual” (document A5E00307987-04, August 2008) and answer the following questions: Locate the section discussing the PLC’s scan cycle and describe the sequence of operations conducted by the PLC on an ongoing basis. Locate the section discussing the PLC’s memory types (“Permanent Memory” versus “Retentive Data Memory” and such), and describe the functions of each.

A very important aspect to learn about any PLC is how to specify various locations within its memory. Each manufacturer and model of PLC has its own way of “addressing” memory locations, analogous to the different ways each postal system within each country of the world specifies its mailing addresses. Locate the section of the manual discussing addressing conventions (“Accessing the Data of the S7-200”), and then answer these questions: Identify the proper address notation for a particular bit in the Siemens PLC’s memory: bit number 4 of byte 1 within the process-image input register. Identify the proper address notation for a particular bit in the Siemens PLC’s memory: bit number 2 of byte 0 within the process-image output register. Identify the proper address notation for a “double word” of data in the Siemens PLC’s memory beginning at byte 105 within the variable memory area. How many bits are contained in a double word? file i03605

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Question 3 Read selected portions of the Allen-Bradley “MicroLogix 1000 Programmable Controllers (Bulletin 1761 Controllers)” user manual (document 1761-6.3, July 1998) and answer the following questions: Locate the section discussing the PLC’s operating cycle – otherwise known as a “scan” cycle – and describe the sequence of operations conducted by the PLC on an ongoing basis. Locate the section discussing the PLC’s memory types (EEPROM and RAM), and describe the functions of each.

A very important aspect to learn about any PLC is how to specify various locations within its memory. Each manufacturer and model of PLC has its own way of “addressing” memory locations, analogous to the different ways each postal system within each country of the world specifies its mailing addresses. Locate the section of the manual discussing addressing conventions (“Addressing Data Files”), and then answer these questions: Identify the proper address notation for a particular bit in the Allen-Bradley PLC’s memory: bit number 4 of element 1 within the input file. Identify the proper address notation for a particular bit in the Allen-Bradley PLC’s memory: bit number 2 of element 0 within the output file. Identify the proper address notation for a “word” of data in the Allen-Bradley PLC’s memory: the accumulator word (ACC) of timer number 6 within data file T4. file i03604

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Question 4 In order to learn PLC programming and perform the exercises necessary for exams in this course, you must have your own PLC trainer consisting of a working PLC and input switches all wired and ready to use. PLC Power I/O

Input switches

Indicator lamps

All components should be securely mounted to a wood board or some other structure making it easy to transport and use. You must have a terminal block in between the switches, indicators, and PLC I/O terminals to allow for easy connection and disconnection of external devices to your PLC without wearing out the screws on the PLC’s terminal block prematurely. Separate terminal blocks are easily replaced, whereas the terminal block on your PLC is likely much more expensive and inconvenient to replace! Consult the user’s manual for your PLC in order to determine how all devices should be wired to the input and output (I/O) terminals. Note that often there are different types of I/O (AC, DC, sourcing, sinking) available for the same (or similar) model of PLC. Most PLC user’s manuals give detailed diagrams showing how to connect devices to discrete I/O points, so be sure to follow the proper diagram for your specific PLC model! Once you have your PLC wired, the next step is to install and run the software used to program your programmable logic controller (PLC), and try to get the two devices communicating with each other. This, of course, requires you have a special cable connecting your PC to your PLC, with any necessary “drivers” installed on your PC to allow it to communicate. Like all serial-based communications, the PC needs to be properly configured with regard to bit rate, number of data bits, number of stop bits, and parity in order to communicate with the PLC. The software you will be using should have an “auto detect” feature which will sequentially try various combinations of these parameters until it finds one combination that works. Note: on Allen-Bradley PLCs, you must first install and run software called RSLinx which manages communications between your PC and PLC, before you start up the programming software (RSLogix). After that, your next step is to use programming software (installed in a personal computer) to program your PLC with some simple function consisting of “contact” and “coil” instructions. The purpose of a virtual contact in a PLC program is to read data bits from memory, while the purpose of a virtual coil in a PLC program is to write data bits to memory. Thus, you will create programs for the PLC using virtual contacts to read the states of real-world switches connected to inputs on the PLC, and using virtual coils to control real-world outputs on the PLC to energize loads such as lamps and solenoids. The interconnections and arrangements of these virtual contacts and coils determine the logic implemented by the PLC: specifying the conditions necessary to energize real-world devices based on input conditions. You will find step-by-step instructional tutorials for both Allen-Bradley MicroLogix and Koyo CLICK PLCs in your Instrumentation Reference (provided by the instructor). Follow these tutorials to establish communication between your PC and your PLC, and to write a simple contact-and-coil ladder diagram program, before attempting the exercises that follow. You will also find much pertinent information for programming Allen-Bradley MicroLogix PLCs in the RSLogix 500 Getting Results Guide, since the SLC 500 24

line of Allen-Bradley PLCs program so similarly to the MicroLogix line. This example shows an Allen-Bradley MicroLogix 1000 series PLC (model 1761-L10BWA) wired to two toggle switches and one LED indicator lamp, complete with a demonstration program. Note that line power (120 VAC) wire connections to power the PLC have been omitted, so the focus is solely on the I/O wiring:

Toggle switch 24V

DC COM

I/0

I/1

I/2

I/3

DC COM

I/4

I/5

DC OUT

Allen-Bradley Power Run

MicroLogix

Fault

1000

Force

85-264 VAC

L1

VAC VDC

L2/N

O/0

VAC VDC

O/1

VAC VDC

O/2

VAC VDC

O/3

LED (with dropping resistor)

Ladder-Diagram program written to PLC: I:0

I:0

O:0

0

1

0 END

Note how Allen-Bradley I/O is labeled in the program: input bits designated by the letter I and output bits designated by the letter O. Based on the wiring and program you see for this PLC, identify the switch state combinations resulting in an energized lamp. Try duplicating this program in your own PLC (even if it is a different brand or model) and see how it functions. Be sure to activate the color highlighting feature of your programming editor so you may see the “live” status of the program’s virtual contacts and coil!

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This example shows a Siemens S7-200 series PLC (model 224XP) wired to two toggle switches and one LED indicator lamp, complete with a demonstration program:

24 VDC

LED (with dropping resistor) SIEMENS

Toggle switch

1M

SIMATIC S7-200

1L+

0.0

0.1

0.2

Q0 SF/DIAG

0.3

0.4

2M

2L+

0.5

0.6

0.7

1.0

1.1

M

DC

CPU 224XP

Q1 .0 .1 .2 .3 .4 .5 .6 .7

L+

DC/DC/DC

.0 .1

RUN STOP

.0 .1 .2 .3 .4 .5 .6 .7 I0

1M Port 1

.0 .1 .2 .3 .4 .5 I1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

2M

1.0

1.1

1.2

1.3

1.4

1.5

M

L+

Port 0

Ladder-Diagram program written to PLC: I0.0

Q0.0

I0.1

END

Note how Siemens I/O is labeled in the program: input bits designated by the letter I and output bits designated by the letter Q. Based on the wiring and program you see for this PLC, identify the switch state combinations resulting in an energized lamp. Try duplicating this program in your own PLC (even if it is a different brand or model) and see how it functions. Be sure to activate the color highlighting feature of your programming editor so you may see the “live” status of the program’s virtual contacts and coil!

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This example shows a Koyo “CLICK” PLC (model C0-02DD1-D) wired to two toggle switches and one LED indicator lamp, complete with a demonstration program:

C0-02DD1-D

CLICK Koyo

C1 X1 X2 X3 X4

PWR RUN ERR

RUN

C2

STOP

Y1

LED (with dropping resistor)

Y2 Y3

PORT 1 TX1

Y4

RX1

+V AD1V

TX2

AD1I

RX2

AD2V

PORT 2

AD2I

Toggle switch

ACOM

PORT 3 RS-485

DA1V DA1I

TX3 RX3

DA2V

LG

0

DA2I

24V

24 VDC

Ladder-Diagram program written to PLC: X1

X2

X2

X1

Y1

END

Note how Koyo I/O is labeled in the program: input bits designated by the letter X and output bits designated by the letter Y. Based on the wiring and program you see for this PLC, identify the switch state combinations resulting in an energized lamp. Try duplicating this program in your own PLC (even if it is a different brand or model) and see how it functions. Be sure to activate the color highlighting feature of your programming editor so you may see the “live” status of the program’s virtual contacts and coil! file i04513

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Question 5 The most basic type of real-world input to a PLC is a discrete (on/off) input. Each discrete input channel on a PLC is associated with a single bit in the PLC’s memory. Use the PLC programming software on your personal computer to “connect” to your PLC, then locate the facility within this software that allows you to monitor the status of your PLC’s discrete input bits. Actuate the switches connected to your PLC’s discrete input channels while watching the status of the respective bits. Based on what you see, what does a “1” bit status signify, and what does a “0” bit status signify? Suggestions for Socratic discussion • How does your PLC address discrete input bits? In other words, what is the convention it uses to label these bits, and distinguish them from each other? • How does the programming software for your PLC provide access to discrete input bit status?

PLC comparison: • Allen-Bradley Logix 5000: the Controller Tags folder (typically on the left-hand pane of the programming window set) lists all the tag names defined for the PLC project, allowing you to view the real-time status of them all. Discrete inputs do not have specific input channel tag names, as tag names are user-defined in the Logix5000 PLC series. • Allen-Bradley PLC-5, SLC 500, and MicroLogix: the Data Files listing (typically on the left-hand pane of the programming window set) lists all the data files within that PLC’s memory. Opening a data file window allows you to view the real-time status of these data points. Discrete inputs are the I file addresses (e.g. I:0/2, I:3/5, etc.). The letter “I” represents “input,” the first number represents the slot in which the input card is plugged, and the last number represents the bit within that data element (a 16-bit word) corresponding to the input card. • Siemens S7-200: the Status Chart window allows the user to custom-configure a table showing the realtime values of multiple addresses within the PLC’s memory. Discrete inputs are the I memory addresses (e.g. I0.1, I1.5, etc.). • Koyo (Automation Direct) DirectLogic and CLICK: the Data View window allows the user to customconfigure a table showing the real-time values of multiple addresses within the PLC’s memory. Discrete inputs are the X memory addresses (e.g. X1, X2, etc.). file i01876 28

Question 6 The most basic type of real-world output from a PLC is a discrete (on/off) output. Each discrete output channel on a PLC is associated with a single bit in the PLC’s memory. Use the PLC programming software on your personal computer to “connect” to your PLC, then locate the facility within this software that allows you to monitor the status of your PLC’s discrete output bits. Use the “force” utility in the programming software to force different output bits to a “1” status. Based on what you see, what does a “1” bit status signify, and what does a “0” bit status signify? Is there any visual indication that bits have been forced from their normal state(s) in your PLC? Note that “forcing” causes the PLC to output the values you specify, whether or not the programming in the PLC “wants” those bits to have those forced values! Suggestions for Socratic discussion • How does your PLC address discrete output bits? In other words, what is the convention it uses to label these bits, and distinguish them from each other? • How does the programming software for your PLC provide access to discrete output bit status, and the ability to force them? • Why would anyone ever wish to force an output bit in a PLC, especially if doing so overrides the logic programmed into the PLC?

PLC comparison: • Allen-Bradley Logix 5000: forces may be applied to specific tag names by right-clicking on the tag (in the program listing) and selecting the “Monitor” option. Discrete outputs do not have specific output channel tag names, as tag names are user-defined in the Logix5000 PLC series. • Allen-Bradley PLC-5, SLC 500, and MicroLogix: the Force Files listing (typically on the left-hand pane of the programming window set) lists those data files within the PLC’s memory liable to forcing by the user. Opening a force file window allows you to view and set the real-time status of these bits. Discrete outputs are the O file addresses (e.g. O:0/7, O:6/2, etc.). The letter “O” represents “output,” the first number represents the slot in which the output card is plugged, and the last number represents the bit within that data element (a 16-bit word) corresponding to the output card. • Siemens S7-200: the Status Chart window allows the user to custom-configure a table showing the realtime values of multiple addresses within the PLC’s memory, and enabling the user to force the values of those addresses. Discrete outputs are the Q memory addresses (e.g. Q0.4, Q1.2, etc.). • Koyo (Automation Direct) DirectLogic and CLICK: the Override View window allows the user to force variables within the PLC’s memory. Discrete outputs are the Y memory addresses (e.g. Y1, Y2, etc.). file i01877 29

Question 7 All PLCs provide “special” locations in memory holding values useful to the programmer, such as status warnings, real-time clock settings, calendar dates, etc. Use the PLC programming software on your personal computer to “connect” to your PLC, then locate the facility within this software that allows you to explore some of these locations in memory. Identify some of the specific status-related and “special” memory locations in your PLC, and comment on those you think might be useful to use in the future. Note the following memory types you may find associated with these addresses: • Boolean (discrete) = simply on or off (1 or 0) • Integer = whole-numbered values • Floating-point (“real”) = fractional values

Suggestions for Socratic discussion • Describe some of the “special” memory locations you find in your search, and comment on how some of them might be useful. • One of the useful bits provided by many PLCs is a “flashing” bit that simply turns on and off at regular intervals. How many of these bits can you find in your PLC’s memory, and how rapidly does each one oscillate?

PLC comparison: • Allen-Bradley Logix 5000: various “system” values are accessed via GSV (Get System Value) and SSV (Save System Value) instructions. • Allen-Bradley PLC-5, SLC 500, and MicroLogix: the Data Files listing (typically on the left-hand pane of the programming window set) shows file number 2 as the “Status” file, in which you will find various system-related bits and registers. • Siemens S7-200: the Special Memory registers contain various system-related bits and registers. These are the SM memory addresses (e.g. SM0.1, SMB8, SMW22, etc.). • Koyo (Automation Direct) DirectLogic and CLICK: the Special registers contain various system-related bits and registers. These are the SP memory addresses (e.g. SP1, SP2, SP3, etc.) in the DirectLogic PLC series, and the SC and SD memory addresses in the CLICK PLC series. file i01878 30

Question 8 Read and outline the “Relating I/O Status to Virtual Elements” subsection of the “Logic Programming” section of the “Programmable Logic Controllers” chapter in your Lessons In Industrial Instrumentation textbook. Note the page numbers where important illustrations, photographs, equations, tables, and other relevant details are found. Prepare to thoughtfully discuss with your instructor and classmates the concepts and examples explored in this reading. The fundamental concept of relating I/O status to program elements is not necessarily easy to grasp. For this reason, a “Process Switches and PLC Circuits” worksheet has been placed in the Socratic Instrumentation practice worksheet collection. Feel free to use this practice worksheet to supplement your studies on this critically important topic! file i04516

31

Question 9 Discrete (on/off) I/O for PLCs often works on AC (alternating current) power. AC input circuitry usually consists of an optocoupler (LED) with rectification and a large dropping resistor to allow 120 volt AC operation. AC output circuitry usually consists of TRIACs. Explain how both of these technologies work. DC I/O for a PLC generally consists of optocoupled LEDs for inputs and bipolar transistors for outputs. Some examples are shown in the following schematics. Note carefully the different variations:

Discrete input module

Discrete input module

X0

Com X0

X1 X1 X2 X2 X3 Com

X3

Discrete output module

Discrete output module

Com Y0

Y0

Y1 Y1 Y2 Y2 Y3 Com

Y3

Determine for each of these input and output module types, whether they would be properly designated sourcing or sinking. Suggestions for Socratic discussion • Determine how real input and output devices (e.g. switches, solenoid coils) would need to be connected to the I/O terminals of these modules. file i02359 32

Question 10 Have some fun writing simple “exploratory” or “demonstration” ladder-diagram PLC programs to perform different functions. Feel free to explore the following instruction types: • Counters (up, down, up/down) • Timers (on-delay, off-delay, retentive) • Sequencing instructions Identify some realistic applications for PLC programs using counters and timers. What sorts of real-life processes might benefit from a PLC function where something turns on or off after a definite number of counts applied to the PLC input, or after a certain amount of time has passed? Note: this simple exercise may seem trivial, but it holds the key to self-instruction on PLC programming! Having your very own PLC to work with in the classroom is a tremendously powerful learning tool. Whenever you encounter a new programming instruction (e.g. a timer, a math instruction, etc.) that you do not yet know how to use, you may explore that instruction’s properties and behavior by creating a simple program in your PLC with nothing but that instruction. Your PLC’s User Manual or Instruction Set reference manual will show you the basic syntax of the instruction, which you may copy verbatim as an example. Once this simple program is loaded into your PLC’s memory, you can “play” with it to see its live behavior while viewing the program online. Once you have directly observed how the instruction works, your next step is to add comments to the program describing how that instruction works in your own words. Be as detailed as possible here, treating this activity as though you were asked to explain everything to someone who knew absolutely nothing about the instruction. These comments will serve as notes to yourself later, when you need to refresh your memory on how a particular instruction functions or what it is used for. Refer to the “Answer” section of this question to see some examples of what such a demonstration program might look like. file i00120

33

Question 11 Write a PLC program that accepts two discrete input signals (from two switches), and outputs the following four discrete outputs: • Output channel #1: The status of input switch #1 (simply repeating input #1) • Output channel #2: The Boolean complement (opposite) of input switch #1 • Output channel #3: The AND function of switches #1 and #2 • Output channel #4: The OR function of switches #1 and #2 Shown here is a generic RLL listing of such a program:

Input_switch_1

Output_1

Input_switch_1

Output_2

Input_switch_1

Input_switch_2

Input_switch_1

Output_3

Output_4

Input_switch_2

Turn on status highlighting within the programming software environment so that you may see the virtual “power” flow through the “conductive” contacts as you test the program.

Suggestions for Socratic discussion • How are discrete input and output points associated with contacts and coils in the ladder-logic program? • How do you draw vertical connecting lines in the ladder-logic program? • How do you assign “alias” names to inputs and outputs for easier program readability? For example, how do you assign an English name to the input I:2/4 (Input channel 4 on card 2) on an Allen-Bradley SLC 500 PLC so that it reads as “Input switch 4” in the program instead of “I:2/4” in the programming software’s display? • Where is the software function (pull-down menu option, button, hot-key, etc.) located that allows you to turn on contact status highlighting in the PLC programming software? file i03667

34

Question 12 Suppose we have an Allen-Bradley MicroLogix 1000 PLC connected to three momentary-contact pushbutton switches as shown in this illustration:

A

24V

DC COM

I/0

I/1

I/2

I/3

DC COM

I/4

I/5

DC OUT

B

Power Run Fault Force

C 85-264 VAC

L1

L2/N

VAC VDC

O/0

VAC VDC

O/1

VAC VDC

O/2

VAC VDC

O/3

Determine the bit statuses of I:0/0, I:0/1, and I:0/2 when switch A is unpressed (released), switch B is unpressed (released), and switch C is pressed. file i01865

35

Question 13 Suppose we have a Siemens S7-200 PLC connected to two process switches as shown in this illustration:

24 VDC

SIEMENS 1M

SIMATIC S7-200

1L+

0.0

0.1

0.2

Q0 SF/DIAG

0.3

0.4

2M

2L+

0.5

0.6

0.7

1.0

1.1

M

DC

CPU 224XP

Q1 .0 .1 .2 .3 .4 .5 .6 .7

L+

DC/DC/DC

.0 .1

RUN STOP

.0 .1 .2 .3 .4 .5 .6 .7 I0

1M Port 1

.0 .1 .2 .3 .4 .5 I1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

2M

1.0

1.1

1.2

1.3

1.4

1.5

M

L+

Port 0

130 oF

12 GPM

Determine the bit statuses of I0.2 and I1.1 when the temperature switch senses 194 o F and the flow switches senses 19 GPM. file i01871

36

Question 14 Suppose we have an Allen-Bradley SLC 500 PLC connected to two process switches as shown in this illustration: Slot 0 (processor)

Power supply

120 VAC power

Processor

Slot 1 Input 0 1 2 3

L1 L2/N Gnd

Slot 2

Slot 3

(discrete input) (discrete input) (discrete output)

Input 4 5 6 7

0 1 2 3

Output 4 5 6 7

0 1 2 3

IN0

IN0

VAC 1

IN1

IN1

OUT0

IN2

IN2

OUT1

IN3

IN3

OUT2

IN4

IN4

OUT3

IN5

IN5

VAC 2

IN6

IN6

OUT4

IN7

IN7

OUT5

COM

COM

OUT6

COM

COM

OUT7

4 5 6 7

3 feet

37 PSI

88 oF

Determine the process conditions necessary to generate the following input bit statuses in the PLC’s memory: • I:1/3 = 1 • I:1/5 = 0 file i01872

37

Question 15 Examine this “live” display of a Siemens S7-300 PLC’s program, and from this determine all bit statuses represented by the color highlighting in this ladder logic program:

I1.1

I0.5

Q0.1

I0.2

I1.1

Q0.6

• I0.2 = ??? • I0.5 = ??? • I1.1 = ??? • Q0.1 = ??? • Q0.6 = ??? file i01873

38

Question 16 Suppose we have a Koyo “CLICK” PLC connected to three momentary-contact pushbutton switches as shown in this illustration: C0-02DD1-D

CLICK Koyo

A

C1 X1 X2

B

X3 X4

PWR RUN ERR

RUN

C2

STOP

Y1

C

Y2 Y3

PORT 1 TX1

Y4

RX1

+V AD1V

TX2

AD1I

RX2

AD2V

PORT 2

AD2I ACOM

PORT 3 RS-485

DA1V DA1I

TX3 RX3

DA2V

LG

0

DA2I

24V

24 VDC Determine the switch actuation statuses (i.e. pressed versus released) given the “live” display of the ladder logic program shown here:

X1

X2

X3

Y1

Also, determine the status of the lamp connected to the PLC’s Y1 output. file i01874

39

Question 17 Explain the function of this light-switching circuit, tracing the directions of all currents when the switch closes:

file i01000 Question 18 A student attempts to build a circuit that will turn a DC motor on and off with a very delicate (low current rating) pushbutton switch. Unfortunately, there is something wrong with the circuit, because the motor does not turn on no matter what is done with the switch:

This circuit does not work!

Mtr

Correct the error(s) in this circuit, showing how it must be set up so that the transistor functions as intended. file i01001

40

Question 19 Some of the following transistor switch circuits are properly configured, and some are not. Identify which of these circuits will function properly (i.e. turn on the load when the switch closes) and which of these circuits are mis-wired:

Circuit 1

Circuit 2

Load

Load

Circuit 3

Circuit 4

Load Load

file i01002

41


Circuit 1

Circuit 2

Load

Load

Circuit 3

Circuit 4

Load Load file i01003

42

Question 21 Read and outline the “Contacts and Coils” subsection of the “Ladder Diagram (LD) Programming” section of the “Programmable Logic Controllers” chapter in your Lessons In Industrial Instrumentation textbook. Note the page numbers where important illustrations, photographs, equations, tables, and other relevant details are found. Prepare to thoughtfully discuss with your instructor and classmates the concepts and examples explored in this reading. Suggestions for Socratic discussion • If you have access to your own PLC for experimentation, I urge you to write a simple demonstration program in your PLC allowing you to explore the behavior of these PLC instructions. The program doesn’t have to do anything useful, but merely demonstrate what each instruction does. First, read the appropriate section in your PLC’s manual or instruction reference to identify the proper syntax for that instruction (e.g. which types of data it uses, what address ranges are appropriate), then write the simplest program you can think of to demonstrate that function in isolation. Download this program to your PLC, then run it and observe how it functions “live” by noting the color highlighting in your editing program’s display and/or the numerical values manipulated by each instruction. After “playing” with your demonstration program and observing its behavior, write comments for each rung of your program explaining in your own words what each instruction does. file i04517

43

Question 22 Suppose a Siemens 545 PLC has the following input bit states: • X1 = 0 • X2 = 1 • X3 = 0 Sketch color highlighting for the contacts and coils in the PLC’s program given these bit statuses, also determining the status of output bit Y1:

X1

X2

X2

X1

X3

Y1

Suggestions for Socratic discussion • PLC training expert Ron Beaufort teaches students to think of a “normally-open” PLC program contact instruction as a command to the PLC’s processor to “Go look for a 1”. Conversely, he teaches students to think of a “normally-closed” instruction as a command to “Go look for a 0”. Explain what Mr. Beaufort means by these phrases, and how this wisdom relates to this particular problem. Incidentally, Mr. Beaufort’s excellent instructional videos (available freely on YouTube) are quite valuable to watch! • Identify the significance of the labels “X” and “Y” for this PLC’s bits. What do you suppose “X” signifies? What do you suppose “Y” signifies? file i04688

44

Question 23 Examine this “live” display of a Siemens S7-300 PLC’s program, and from this determine all bit statuses represented by the color highlighting in this ladder logic program:

I1.1

I0.7

Q0.1

I0.7

I1.1

Q0.3

• I0.7 = ??? • I1.1 = ??? • Q0.1 = ??? • Q0.3 = ???

Suggestions for Socratic discussion • PLC training expert Ron Beaufort teaches students to think of a “normally-open” PLC program contact instruction as a command to the PLC’s processor to “Go look for a 1”. Conversely, he teaches students to think of a “normally-closed” instruction as a command to “Go look for a 0”. Explain what Mr. Beaufort means by these phrases, and how this wisdom relates to this particular problem. Incidentally, Mr. Beaufort’s excellent instructional videos (available freely on YouTube) are quite valuable to watch! • Identify the significance of the labels “I” and “Q” for this PLC’s bits. What do you suppose “I” signifies? What do you suppose “Q” signifies? file i04689

45

Question 24 Suppose we have an Allen-Bradley model “SLC 500” PLC connected to a pair of momentary-contact pushbutton switches and light bulbs as shown in this illustration:

Power supply

Slot 0

Slot 1

Slot 2

Slot 3

(processor)

(discrete input)

(unused)

(discrete output)

Processor

Input 0 1 2 3

L1

120 VAC power

L2/N Gnd

Output 0 1 2 3

4 5 6 7

IN0

VAC 1

IN1

OUT0

IN2

OUT1

IN3

OUT2

IN4

OUT3

IN5

VAC 2

IN6

OUT4

IN7

OUT5

COM

OUT6

COM

OUT7

4 5 6 7

Switch A

Lamp Y

Switch B

Lamp Z

Examine the following relay ladder logic (RLL) program for this Allen-Bradley PLC, determining the statuses of the two lamps provided neither switch A nor switch B is pressed by a human operator:

I:1

I:1

O:3

2

6

0

I:1

I:1

O:3

2

6

4

Finally, draw color highlighting showing how these “contact” instructions will appear in an online editor program given the stated input conditions. Suggestions for Socratic discussion • Identify the significance of the labels “I” and “O” for this PLC’s bits. • Identify the significance of the first and second numbers in each bit label (e.g. the numbers “1” and “2” in the bit address I:1/2, for example). What pattern do you see as you compare the I/O connections with the respective contact instructions in the PLC program? file i04628

46

Question 25 Suppose we have a Siemens S7-200 PLC connected to a pair of momentary-contact pushbutton switches and light bulbs as shown in this illustration:

24 VDC

SIEMENS 1M

SIMATIC S7-200

1L+

0.0

0.1

0.2

0.3

2M

2L+

0.5

0.6

0.7

1.0

1.1

M

.0 .1 .2 .3 .4 .5 .6 .7

L+

DC

CPU 224XP

Q1

Q0 SF/DIAG

0.4

DC/DC/DC

.0 .1

RUN STOP

.0 .1 .2 .3 .4 .5 .6 .7

1M Port 1

.0 .1 .2 .3 .4 .5 I1

I0

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

2M

1.0

1.1

1.2

1.3

1.4

1.5

M

L+

Port 0

Lamp Y

Switch A

Lamp Z

Switch B

Examine the following relay ladder logic (RLL) program for this Siemens PLC, determining the statuses of the two lamps provided both switches are simultaneously pressed by a human operator:

I1.2

I0.7

Q0.1

I0.7

I1.2

Q0.3

Finally, draw color highlighting showing how these “contact” instructions will appear in an online editor program given the stated input conditions. file i04664

47

Question 26 Suppose we have a Koyo “CLICK” PLC connected to three momentary-contact pushbutton switches as shown in this illustration: C0-02DD1-D

CLICK Koyo

C1

A

X1 X2 X3

B

X4

PWR RUN ERR

RUN

C2

STOP

Y1 Y2

C

Y3

PORT 1 TX1

Y4

RX1

+V AD1V

TX2

AD1I

RX2

AD2V

PORT 2

AD2I ACOM

PORT 3 RS-485

DA1V DA1I

TX3 RX3

DA2V

LG

0

DA2I

24V

24 VDC Determine the necessary switch actuation statuses (i.e. pressed versus unpressed) to turn the lamp on given the following program running in the PLC:

X1

X3

Y1

X2

Suggestions for Socratic discussion • Identify the significance of the labels “X” and “Y” for this PLC’s bits. What do you suppose “X” signifies? What do you suppose “Y” signifies? file i04638

48

Question 27 Suppose we have an Allen-Bradley MicroLogix 1000 controller connected to a pair of momentary-contact pushbutton switches and contactor controlling power to an electric motor as shown in this illustration:

"Start" switch

24V

DC COM

I/0

I/1

I/2

I/3

DC COM

I/4

I/5

DC OUT

"Stop" switch


OL contact 85-264 VAC

L1

L2/N

VAC VDC

O/0

VAC VDC

O/1

VAC VDC

O/2

VAC VDC

O/3

Contactor coil

This motor control system has a problem, though: the motor refuses to start when the “Start” pushbutton is pressed. Examine the “live” display of the ladder logic program inside this Allen-Bradley PLC to determine what the problem is, assuming an operator is continuously pressing the “Start” pushbutton as you examine the program:

I:0/3

I:0/2

I:0/0

O:0/2

O:0/2

Identify at least two causes that could account for all you see here. Suggestions for Socratic discussion • Identify what your next troubleshooting step would be if you were tasked with solving this problem. • A helpful problem-solving tip is to annotate each contact in the PLC program to show what its realworld function is. For example, contact I:0/3 may be labeled “OL” because that is the real-world switch status it senses. Annotate all contacts in this program and explain how this annotation is helpful in analyzing the program. • Describe the purpose of the contact labeled O:0/2 in this program, explaining why it is often referred to as a seal-in contact. 49

file i04662 Question 28 Suppose we have an Allen-Bradley SLC 500 controller connected to a pair of momentary-contact pushbutton switches and contactor controlling power to an electric motor as shown in this illustration:

480 VAC

Power supply 1

X2

H2

3

H3

4

F7

H1

L1 L2/N

F5

F1

F2

F3

Input 0 1 2 3

X1

F6 H4

2

Processor

Gnd

Output 4 5 6 7

0 1 2 3

Input 4 5 6 7

Start

Analog

IN0

VAC 1

IN 0+ IN 0-

IN1

OUT0

IN2

OUT1

IN3

OUT2

IN4

OUT3

IN5

VAC 2

IN 2+ IN 2-

IN6

OUT4

ANL COM

IN7

OUT5

COM

OUT6

IN 3+ IN 3-

COM

OUT7

ANL COM

IN 1+ IN 1ANL COM

Stop

ANL COM

5

F4

6 7 8

Contactor T3 T2 T1

Overload block

Reset

Motor

This motor control system has a problem, though: the motor refuses to start when the “Start” pushbutton is pressed. Closely examine the pictorial diagram, then identify at least two faults that could account for the motor’s refusal to start. Suggestions for Socratic discussion • A helpful problem-solving tip is to note the PLC’s I/O states by examining the LED indicators on each input and output card on the PLC rack. What do the LED states tell you in this particular example? file i04069

50

Question 29 Two technicians, Jill and Bob, work on programming Siemens S7-200 PLCs to control the starting and stopping of electric motors. Both PLCs are wired identically, as shown:

120 VAC supply

480 VAC 3-θ supply

SIEMENS 1M 1L

SIMATIC S7-200

1L+ 0.0

0.1 0.0

0.2 0.1

0.3 0.2

Q0 SF/DIAG

0.3

0.4 2L

0.4 2M

0.5 2L+

0.5 0.6

0.6

0.7 3L

1.0 0.7

1.1 1.0

1.1

N M

CPU 224XP

Q1 .0 .1 .2 .3 .4 .5 .6 .7

L+ L1 AC DC

DC/DC/DC AC/DC/Relay

.0 .1

RUN STOP

.0 .1 .2 .3 .4 .5 .6 .7 I0

1M Port 1

.0 .1 .2 .3 .4 .5 I1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

2M

1.0

1.1

1.2

1.3

1.4

1.5

M

L+

Port 0

Start

Stop

However, despite being wired identically, the two technicians’ PLC programs are quite different. Jill’s program uses retentive coil instructions (“Set” and “Reset” coils) while Bob’s uses a “seal-in” contact instruction to perform the function of latching the motor on and off: Jill’s PLC program I0.1

Bob’s PLC program

Q0.0

I0.1

I0.4

Q0.0

S I0.4

Q0.0

Q0.0

R

Explain how both of these PLC programs function properly to control the starting and stopping of the electric motor. Suggestions for Socratic discussion • It is ordinarily a bad thing to assign identical bit addresses to multiple coil instructions in a PLC program. With Jill’s retentive coil program, however, this is not only permissible but in fact necessary for its proper operation. Explain why this is. • A common misconception of students first learning PLC programming is to think that the type of contact instruction used in the PLC program must match the type of switch contact connected to that input (e.g. “A N.O. PLC instruction must go with a N.O. switch”). Explain why this is incorrect. file i03674

51

Question 30 Programming Challenge and Comparison – Conveyor start/stop control with safety switch Suppose we wish to control the starting and stopping of a large conveyor belt at a factory using a PLC. This control system will use a “Start” pushbutton, a “Stop” pushbutton, and an emergency shut-down pull-cable allowing anyone along the conveyor’s length to stop the belt simply by tugging on a steel cable (this is akin to the “stop” cable used on public buses for passengers to signal to the driver their intent to get off at the next stop). Inputs • Start pushbutton (momentary NO) – pushing this button closes the switch to energize the PLC input • Stop pushbutton (momentary NC) – pushing this button opens the switch to de-energize the PLC input • Emergency stop cable (latching NC) – tugging on the cable opens the switch to de-energize the PLC input Outputs • Motor contactor – energizing this PLC output starts the conveyor belt motor Work individually or in teams to write a PLC program performing this function, and demonstrate its operation using switches connected to its inputs to simulate the discrete inputs in a real application. When your program is complete and tested, capture a screen-shot of it as it appears on your computer, and prepare to present your program solution to the class in a review session for everyone to see and critique. The purpose of this review session is to see multiple solutions to one problem, explore different programming techniques, and gain experience interpreting PLC programs others have written. When presenting your program (either individually or as a team), prepare to discuss the following points: • Identify the “tag names” or “nicknames” used within your program to label I/O and other bits in memory • Follow the sequence of operation in your program, simulating the system in action • Identify any special or otherwise non-standard instructions used in your program, and explain why you decided to take that approach • Show the comments placed in your program, to help explain how and why it works • How you designed the program (i.e. what steps you took to go from a concept to a working program)

Suggestions for Socratic discussion • How do you keep the motor “latched” on when the momentary “Start” switch is released? • Which is simpler: implementing this function using normal program coils, or implementing this function using retentive coils (“set” and “reset”, or “latch” and “unlatch”)?

file i02340 52

Question 31 Suppose we have an Allen-Bradley MicroLogix 1000 PLC connected to three momentary-contact pushbutton switches as shown in this illustration:

A

24V

DC COM

I/0

I/1

I/2

I/3

DC COM

I/4

I/5

DC OUT

B


C 85-264 VAC

L1

L2/N

VAC VDC

O/0

VAC VDC

O/1

VAC VDC

O/2

VAC VDC

O/3

Determine the bit statuses of I:0/0, I:0/1, and I:0/3 when switch A is pressed, switch B is unpressed (released), and switch C is pressed. file i04685

53

Question 32 Suppose we have a Siemens S7-200 PLC connected to two process switches as shown in this illustration:

24 VDC

SIEMENS 1M

SIMATIC S7-200

1L+

0.0

0.1

0.2

Q0 SF/DIAG

0.3

0.4

2M

2L+

0.5

0.6

0.7

1.0

1.1

M

DC

CPU 224XP

Q1 .0 .1 .2 .3 .4 .5 .6 .7

L+

DC/DC/DC

.0 .1

RUN STOP

.0 .1 .2 .3 .4 .5 .6 .7 I0

1M Port 1

.0 .1 .2 .3 .4 .5 I1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

2M

1.0

1.1

1.2

1.3

1.4

1.5

M

L+

Port 0

130 oF

12 GPM

Determine the bit statuses of I0.2 and I1.1 when the temperature switch senses 122 o F and the flow switches senses 15 GPM. file i04686

54

Question 33 Suppose we have an Allen-Bradley SLC 500 PLC connected to two process switches as shown in this illustration: Slot 0 (processor)

Power supply

120 VAC power

Processor

Slot 1 Input 0 1 2 3

L1 L2/N Gnd

Slot 2

Slot 3

(discrete input) (discrete input) (discrete output)

Input 4 5 6 7

0 1 2 3

Output 4 5 6 7

0 1 2 3

IN0

IN0

VAC 1

IN1

IN1

OUT0

IN2

IN2

OUT1

IN3

IN3

OUT2

IN4

IN4

OUT3

IN5

IN5

VAC 2

IN6

IN6

OUT4

IN7

IN7

OUT5

COM

COM

OUT6

COM

COM

OUT7

4 5 6 7

2 feet

37 PSI

Determine the bit statuses of I:1/3 and I:1/5 when the level switch senses 3 feet and the pressure switch senses 14 PSI. file i04687

55

Question 34 The following PLC program preforms the function of an alarm annunciator, where a discrete input signal from an alarm switch (e.g. high temperature alarm) first causes a warning light to blink and a siren to audibly pulse until a human operator presses an acknowledge pushbutton. If the alarm switch signal is still activated, the light will remain on (steady) instead of blink and the siren will go silent. The light turns off as soon as the alarm signal goes back to its “safe” state. A timing diagram shows how this should work:

Alarm switch Warning light Warning siren Acknowledge pushbutton

Alarm_input

Blink

Light

Latch

Blink

Latch

Acknowledge_input Alarm_input

Siren

Latch

Latch

Take this “generic” PLC program and enter it into your own PLC, assigning appropriate addresses to all instructions, and demonstrating its operation. Suggestions for Socratic discussion • Does the PLC program (as written) “expect” a closed alarm switch contact to trigger the alarm, or an open alarm switch contact? file i02342

56


Circuit 1

Circuit 2

Circuit 3

Circuit 4

Circuit 5

Circuit 6

file i01004

57

Question 36 In each of the following circuits, the light bulb will energize when the pushbutton switch is actuated. Assume that the supply voltage in each case is somewhere between 5 and 30 volts DC (with lamps and resistors appropriately sized):

Circuit 1

Circuit 2

Circuit 3

Circuit 4

Circuit 5

Circuit 6

However, not all of these circuits are properly designed. Some of them will function perfectly, but others will function only once or twice before their transistors fail. Identify the faulty circuits, and explain why they are flawed. file i01005

58

Question 37 Draw the necessary wire connections so that bridging the two contact points with your finger (creating a high-resistance connection between those points) will turn the light bulb on:

Contact points

file i01006 Question 38 Choose the right type of bipolar junction transistor for each of these switching applications, drawing the correct transistor symbol inside each circle:

+V

+V

+V

Load Switch sourcing current to transistor

Switch sinking current from transistor

Transistor sourcing current to load

Load

file i01007

59

Transistor sinking current from load

Question 39 Choose the right type of bipolar junction transistor for each of these switching applications, drawing the correct transistor symbol inside each circle:

+V

+V

+V Load

Switch sourcing current to transistor




Load

Also, explain why resistors are necessary in both these circuits for the transistors to function without being damaged. file i01008 Question 40

60

Question 41 Suppose we have a Siemens S7-200 PLC connected to a pair of momentary-contact pushbutton switches and light bulbs as shown in this illustration:

24 VDC

SIEMENS 1M

SIMATIC S7-200

1L+

0.0

0.1

0.2

0.3

2M

2L+

0.5

0.6

0.7

1.0

1.1

M

.0 .1 .2 .3 .4 .5 .6 .7

L+

DC

CPU 224XP

Q1

Q0 SF/DIAG

0.4

DC/DC/DC

.0 .1

RUN STOP

.0 .1 .2 .3 .4 .5 .6 .7

1M Port 1

.0 .1 .2 .3 .4 .5 I1

I0

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

2M

1.0

1.1

1.2

1.3

1.4

1.5

M

L+

Port 0

Lamp Y

Switch A

Lamp Z

Switch B

Examine the following relay ladder logic (RLL) program for this Siemens PLC, determining the statuses of the two lamps provided switch A is pressed by a human operator and switch B is unpressed:

I1.2

I0.7

Q0.1

I0.7

I1.2

Q0.3

Furthermore, determine whether the inputs and outputs of this particular PLC (as shown) are sourcing or sinking. file i04170

61

Question 42 Suppose we have a Koyo “CLICK” PLC connected to three process switches as shown in this illustration: C0-02DD1-D

CLICK Koyo

30 PSI

C1 X1 X2 X3

150 oF

X4

PWR RUN ERR

RUN

C2

STOP

Y1

4 inches

Y2 Y3

PORT 1 TX1

Y4

RX1

+V AD1V

TX2

AD1I

RX2

AD2V

PORT 2

AD2I ACOM

PORT 3 RS-485

DA1V DA1I

TX3 RX3

DA2V

LG

0

DA2I

24V

24 VDC Determine the switch stimuli (i.e. required pressure, temperature, and level) given the “live” display of the ladder logic program shown here:

X1

X2

X3

Y1

Also, determine the status of the lamp connected to the PLC’s Y1 output. Suggestions for Socratic discussion • Identify how you could force the lamp to energize, if your only tool at hand was a screwdriver. file i04667

62

Question 43 This Koyo “CLICK” PLC has been programmed to control the starting and stopping of an electric motor, including a counter instruction to prevent the motor from being started up more than a specified number of times: C0-02DD1-D

CLICK Koyo

Start

Program (inside PLC)

C1 X1

X1

X2

X2

CT1

Y1

Stop

X3 X4

PWR

RUN

RUN ERR

C2 Y1

STOP

Reset

Y2 PORT 1

Y3

TX1

Y4

RX1

+V

Y1

AD1V TX2

AD1I

RX2

AD2V

PORT 2

AD2I ACOM

PORT 3 RS-485

M1

Y1

Contactor relay coil

Up

DA1V

TX3

DA1I

RX3

DA2V

LG

X3

Counter

CT1

SetPoint

8

Current

CTD1

CT1 Complete

DA2I Reset

0

24V

24 VDC

Identify the counter instruction in the program shown, its input “connections”, and also how the result of the counter reaching its pre-set limit forces the motor to stop. Also, determine the maximum number of times the motor may be started up, assuming the counter’s current value goes to zero when the Reset button is pressed. Finally, determine how to modify this PLC program so that the counter may be manually reset by the operator without requiring a separate pushbutton labeled “Reset”. Suggestions for Socratic discussion • If an operator presses the “Start” button multiple times while the motor is already running, do these button-presses get counted by the counter instruction, or do only the real motor start-up events get counted? • What do you suppose the label “CTD1” represents inside the counter instruction? • Note the number of times the bit Y1 is referenced inside this PLC program: once in a coil instruction and twice in contact instructions. Is there any limit to how many times a bit address may be used in a PLC program? • Describe the purpose of the first contact instruction labeled Y1 in this program, explaining why it is often referred to as a seal-in contact. file i03589

63

Question 44 Read selected portions of the Siemens “SIMATIC S7-200 Programmable Controller System Manual” (document A5E00307987-04, August 2008) and answer the following questions: Identify the different types of SIMATIC counter instructions. Identify a practical application for a counter instruction programmed into a PLC. How high can one of these counter instructions count up to? How low can it count down to? Based on these values, how many bits do you think are used in the register to store a counter instruction’s current value? Sketch a simple ladder-diagram program for a Siemens S7-200 PLC whereby a switch connected to input I0.5 causes a counter to increment (count up) and then turn on an alarm light output Q0.3 when the count reaches a value of 5. Also provide a “reset” function triggered by a normally-open switch contact at input I0.0 to force the count value back to zero when pressed. Suggestions for Socratic discussion • If you have access to your own PLC for experimentation, I urge you to write a simple demonstration program in your PLC allowing you to explore the behavior of these PLC instructions. The program doesn’t have to do anything useful, but merely demonstrate what each instruction does. First, read the appropriate section in your PLC’s manual or instruction reference to identify the proper syntax for that instruction (e.g. which types of data it uses, what address ranges are appropriate), then write the simplest program you can think of to demonstrate that function in isolation. Download this program to your PLC, then run it and observe how it functions “live” by noting the color highlighting in your editing program’s display and/or the numerical values manipulated by each instruction. After “playing” with your demonstration program and observing its behavior, write comments for each rung of your program explaining in your own words what each instruction does. file i02245

64

Question 45 Read selected portions of the Allen-Bradley “Logix5000 Controllers General Instructions” reference manual (publication 1756-RM0031-EN-P, January 2007) and answer the following questions: Identify the different types of counter instructions offered in the Logix5000 PLC family. How high can one of these counter instructions count up to? How low can it count down to? Based on these values, how many bits do you think are used in the register to store a counter instruction’s current value? Unlike the Siemens S7 family of PLCs, the Allen-Bradley counter instruction “box” symbols do not provide a place to connect a reset input. How then is it possible to command a counter instruction to reset back to zero? Sketch a simple ladder-diagram program for an Allen-Bradley Logix5000 PLC whereby a hightemperature switch input with the tag-name High Motor Temp causes a counter to increment (count up) every time a motor overheats, and then turn on an alarm light output (tag-name Alarm Lamp) when the count reaches a value of 5. Also provide a “reset” function triggered by a normally-open switch contact (tag-name Alarm Reset) to force the count value back to zero when pressed. Suggestions for Socratic discussion • If you have access to your own PLC for experimentation, I urge you to write a simple demonstration program in your PLC allowing you to explore the behavior of these PLC instructions. The program doesn’t have to do anything useful, but merely demonstrate what each instruction does. First, read the appropriate section in your PLC’s manual or instruction reference to identify the proper syntax for that instruction (e.g. which types of data it uses, what address ranges are appropriate), then write the simplest program you can think of to demonstrate that function in isolation. Download this program to your PLC, then run it and observe how it functions “live” by noting the color highlighting in your editing program’s display and/or the numerical values manipulated by each instruction. After “playing” with your demonstration program and observing its behavior, write comments for each rung of your program explaining in your own words what each instruction does. file i02664

65

Question 46 This Siemens S7-200 PLC is supposed to count the number of cars entering a parking garage, using a pressure-sensitive switch that the cars drive over when entering the garage. The car-count value is sent to a computer in the main office via a network cable plugged into the PLC. The parking attendant is able to reset the count to 0 at the end of his shift, using a key-switch:

...

Network cable to main office display

SIEMENS 1M

SIMATIC S7-200

1L+

0.0

0.1

0.2

Q0 SF/DIAG

0.3

0.4

2M

2L+

0.5

0.6

0.7

1.0

1.1

M

DC

CPU 224XP

Q1 .0 .1 .2 .3 .4 .5 .6 .7

L+

DC/DC/DC

.0 .1

RUN STOP

.0 .1 .2 .3 .4 .5 .6 .7 I0

1M Port 1

.0 .1 .2 .3 .4 .5 I1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

2M

1.0

1.1

1.2

1.3

1.4

1.5

M

L+

Port 0

Drive-over pressure switch Key Reset switch

Unfortunately, there is something wrong with this system. Although it worked just fine yesterday, today the counter’s current value as displayed on the main office computer seems to be stuck at 574 no matter how many more cars drive over the pressure switch and enter the garage. Explain how you would go about diagnosing the problem in this system, justifying each step you would take. Suggestions for Socratic discussion • A useful troubleshooting strategy is to mentally divide this system into three major portions, and try to determine which portion the problem lies within: (1) the switches and wiring connected to the PLC, (2) the PLC itself, and (3) the network cable and computer in the main office. • How important is the fact that this system worked fine yesterday? Does this knowledge help you in your troubleshooting? • Are there any LED indicators on the face of the PLC that might be helpful in providing diagnostic data for you to pinpoint the location of the problem? file i03683

66

Question 47 This Siemens S7-200 PLC has been programmed to count the number of people in a room, by incrementing a counter every time a person enters through the doorway, and decrementing that same counter whenever someone exits through the same doorway. The two optical switches activate whenever their respective light beams are broken by someone passing through. Their horizontal separation is just a couple of inches – much less than the girth of a person’s torso. The operating status of each switch is that it energizes the PLC input when the light beam is broken:

Light sources

Entering

Photo-switches PLC SIEMENS 1M

SIMATIC S7-200

1L+

0.0

0.1

0.2

Q0 SF/DIAG

0.3

0.4

2M

2L+

0.5

0.6

0.7

1.0

1.1

M

L+

DC

CPU 224XP

Q1 .0 .1 .2 .3 .4 .5 .6 .7

DC/DC/DC

.0 .1

RUN STOP

.0 .1 .2 .3 .4 .5 .6 .7 I0

1M Port 1

.0 .1 .2 .3 .4 .5 I1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

2M

1.0

1.1

1.2

1.3

1.4

1.5

M

L+

Port 0

Examine the program in this PLC for counting people, and determine how it is able to differentiate between a person entering the room and a person leaving the room: I1.0

I1.3 P

I1.3

CU

CTUD

I1.0 P

CD

QU

R

QD

LD

PV

CV

Suggestions for Socratic discussion • Explain how a timing diagram of the switch states would be helpful in analyzing the operation of this PLC program. • How are transition (edge-detecting) instructions implemented in Allen-Bradley PLCs? file i00185 67

Question 48 A PLC is used to count the number of cans traveling by on a conveyor belt in a fish canning factory. An optical proximity switch detects the passage of each can, sending a discrete (on/off) signal to one of the PLC’s input channels. The PLC then counts the number of pulses to determine the number of cans that have passed by:

Power supply

Input

Processor

0 1 2 3

Input 4 5 6 7

DC sourcing

24 VDC

L1 120 VAC

L2/N Gnd

2 3 4

DC sinking

Output 4 5 6 7

0 1 2 3

DC sinking

IN0

IN0

VDC

IN1

IN1

OUT0

IN2

IN2

OUT1

IN3

IN3

OUT2

IN4

IN4

OUT3

IN5

IN5

OUT4

IN6

IN6

OUT5

IN7

IN7

OUT6

COM

COM

OUT7

COM

COM

COM

TB-20

TB-23 1

0 1 2 3

Grn

4 5 6 7

Cable 45

1

Red

2

Blu

3

Org

4

Counter reset

TB-31

1

2

3

4

Wht Blk

Red

DC sinking

One day the canning line operator tells you the PLC has stopped counting even though cans continue to run past the proximity switch as the conveyor belt moves. Identify what you would do to begin diagnosing this problem, justifying each step you would take. Suggestions for Socratic discussion • Identify different areas or components within this system that could possibly be at fault, as a prelude to identifying specific diagnostic steps. • Are there any ways you could diagnose this problem without the use of test equipment (e.g. multimeter)? • Explain the significance of the “sourcing” and “sinking” labels on the I/O cards as well as the proximity switch. file i02428

68

Question 49 The manufacturing company you work for installs a PLC control system on its assembly line, counting the number of components produced every shift. For quite a while, the system works without any problems whatsoever, and then one day management decides to scrap a run of product mid-shift and start over. This is when they discover the system integrator they contracted to build and program the PLC system provided no way to reset the shift production counter except to wait until the shift is over. An operations manager summons you to reset the counter for them. Identify at least two different ways you could reset the counter to meet their needs, as quickly as possible. file i00182

69

Question 50 A PLC is being used to monitor the oil pressure for a steam turbine driving an electrical generator, shutting steam off to the turbine if ever the oil pressure drops below a 10 PSI limit. The turbine’s lubrication oil pump is driven by the turbine shaft itself, supplying itself with pressurized lubricating oil to keep all the turbine bearings properly lubricated and cooled: Start PLC

PSL

Stop

S

Oil pump

Turbine

Generator

20 PSI air supply (vent)

ATO

Steam supply

Another technician programmed the PLC for the start/stop function, but this program has a problem:

Real-world I/O wiring Discrete input card

Discrete output card

"Start" pushbutton

IN_switch_Start

Solenoid coil OUT_valve

"Stop" pushbutton IN_switch_Stop

Low oil pressure IN_oil_press

PLC program IN_switch_Start

IN_switch_Stop

IN_oil_press

OUT_valve

OUT_valve

Identify what this problem is, and fix it! Hint: the oil pump is driven by the turbine, and as such cannot generate any oil pressure until the turbine begins to spin. file i00189

70

Question 51 An important pump in a chemical process is turned by an electric motor, and operators want to have visual indication in the control room that the pump is indeed turning. There is no way to attach a speed switch to the pump shaft (that would be too easy!). Instead, someone has installed a proximity switch near the pump shaft, situated to pick up the passing of a keyway in the shaft with each rotation. Thus, the proximity switch will output a “pulse” signal when the pump shaft is spinning:

Motor Pump

Proximity switch

Signal cable to PLC input I:3/2

Pulse signal (when pump is running)

Operators wanted the indicator light in the control room to blink when the pump is running, for an indication of shaft motion. The problem is, the shaft turns much too fast to directly drive the indicator with the proximity switch signal, and so an Allen-Bradley PLC was programmed to produce a slower blink using this program:

I:3/2

CTU Count Up Counter Preset Accum

C5:0.ACC/13

CU

C5:0

DN

32767 0 O:1/5

C5:0/DN

C5:0 RES

Explain how this program works to fulfill the function of a frequency divider, converting the high-speed pulse signal of the proximity switch into a low-speed blink for the operator light. Suggestions for Socratic discussion • Explain how a frequency divider circuit built out of J-K flip-flop integrated circuits functions, and then describe how this PLC program is similar in principle. file i03838 71

Question 52 A NAND logic function may be built up from a regular AND function plus an inverter function (a NOT gate) on the output:

AND

NAND

NOT . . . is equivalent to . . .

The same strategy of “building” a NAND gate may be done in PLC ladder-diagram programming, by combining a normally-closed contact instruction with two contacts in series. Examine these two Allen-Bradley PLC programs, and explain why the left-hand program is “wasteful” while the right-hand program makes more efficient use of available bits:

Wasteful I:0/4

I:0/7

O:2/0

Allen-Bradley MicroLogix/SLC Efficient O:2/0

I:0/4

O:2/1

B3:0/0

I:0/7

B3:0/0

O:2/1

Examine these two Siemens S7 PLC programs, and explain why the left-hand program is “wasteful” while the right-hand program makes more efficient use of available bits in the same ways the Allen-Bradley example programs were wasteful/efficient:

Siemens Step 7 (S7) Wasteful I0.4

Q2.0

I0.7

Efficient Q2.0

I0.4

Q2.1

M0.0

I0.7

M0.0

Q2.1

Note: many novice PLC programmers commit this error of “wasting” valuable I/O as they write their programs! file i04092

72

Question 53 Programming Challenge and Comparison – Mixer motor auto-stop A batch mixing process in a manufacturing facility uses a mixer motor and a large “paddlewheel” to mix liquid ingredients to make a final product. A PLC needs to run this motor for exactly 1500 turns of the paddlewheel and then automatically stop. The motor needs to be able to start back up if the “Start” button is pressed again for the next batch: Power cable

Liquid

Prox. switch

Mixing vessel

Sensor cable (to PLC)

Paddlewheel

Inputs • Start pushbutton (momentary NO) – pushing this button closes the switch to energize the PLC input • Stop pushbutton (momentary NC) – pushing this button opens the switch to de-energize the PLC input • Proximity switch (NO) – one pulse per paddle revolution Outputs • Motor contactor – energizing this PLC output starts the mixing motor Work individually or in teams to write a PLC program performing this function, and demonstrate its operation using switches connected to its inputs to simulate the discrete inputs in a real application. When your program is complete and tested, capture a screen-shot of it as it appears on your computer, and prepare to present your program solution to the class in a review session for everyone to see and critique. The purpose of this review session is to see multiple solutions to one problem, explore different programming techniques, and gain experience interpreting PLC programs others have written. When presenting your program (either individually or as a team), prepare to discuss the following points: • Identify the “tag names” or “nicknames” used within your program to label I/O and other bits in memory • Follow the sequence of operation in your program, simulating the system in action • Identify any special or otherwise non-standard instructions used in your program, and explain why you decided to take that approach • Show the comments placed in your program, to help explain how and why it works • How you designed the program (i.e. what steps you took to go from a concept to a working program)

Suggestions for Socratic discussion • How can you test your program’s basic operation without having to flick a switch 1500 times to simulate the full number of paddle revolutions? • How can we design the program so that pressing the “Start” button does not influence the count during a run, but only resets the count after the run has completed? file i03688 73


CLICK Koyo

C1

Trip = 135 oF

X1 X2 X3 X4

PWR RUN ERR

RUN

C2

STOP

Y1

Trip = 23 inches

Y2 Y3

PORT 1 TX1

Y4

RX1

+V

Trip = 17 PSI

AD1V TX2

AD1I

RX2

AD2V

PORT 2

AD2I ACOM

PORT 3 RS-485

DA1V DA1I

TX3 RX3

DA2V

LG

0

DA2I

24V

24 VDC Determine the process conditions (i.e. temperature, level, and pressure values) given the “live” display of the ladder logic program shown here:

X1

X2

X3

Y1

Also, determine the status of the lamp connected to the PLC’s Y1 output. file i02144

74


CLICK Koyo

C1 X1

Trip = 32 PSI

X2 X3 X4

PWR RUN ERR

RUN

C2

STOP

Y1

Trip = 10 inches

Y2 PORT 1

Y3

TX1

Y4

RX1

+V

Trip = 99 oF

AD1V TX2

AD1I

RX2

AD2V

PORT 2

AD2I ACOM

PORT 3 RS-485

DA1V DA1I

TX3 RX3

DA2V

LG

0

DA2I

24V

24 VDC Determine the process conditions (i.e. temperature, level, and pressure values) given the “offline” display of the ladder logic program shown here, knowing that the lamp happens to be energized at the present time:

X1

X2

X3

X2

Y1

file i02145

75

Question 56 Programming Challenge – Hour/Minute/Second timer Many PLCs provide a range of special contacts to the programmer. Among these “special contacts” is typically one that cycles on and off at a rate of once per second, like a 1 Hz clock pulse. Research the special contact for this function in your PLC, then write a PLC program for an Hour/Minute/Second timer using three counter instructions: one to count seconds (0 to 59), one to count minutes (0 to 59), and one to count hours. Suggestions for Socratic discussion • What is the address of the special contact in your PLC for the 1 Hz clock pulse? • How do you make three counters count in the correct sequence, so that one represents seconds, the next minutes, and the next hours?

PLC comparison: • Allen-Bradley SLC 500: status bit S:4/0 is a free-running clock pulse with a period of 20 milliseconds, which may be used to clock a counter instruction up to 50 to make a 1-second pulse (because 50 times 20 ms = 1000 ms = 1 second). • Siemens S7-200: Special Memory bit SM0.5 is a free-running clock pulse with a period of 1 second. • Koyo (Automation Direct) DirectLogic: Special relay SP4 is a free-running clock pulse with a period of 1 second. file i03691 76

Question 57 Programming Challenge – Parking garage counter Suppose we wish to count the number of cars inside a parking garage at any given time, by incrementing a counter each time a car enters the garage through the entry lane, and decrementing the same counter each time a car leaves the garage through the exit lane. One discrete input of the PLC will connect to a switch detecting the passing of each car through the garage entry, and another discrete input of the PLC will connect to a switch detecting cars passing out the garage exit. The PLC must be equipped with a way to for the garage attendant to manually reset the counter to zero. Write a PLC program to perform this function, and demonstrate its operation using switches connected to its inputs to simulate the discrete inputs in a real application. Suggestions for Socratic discussion • What type of switches would you recommend to detect cars driving into the parking garage? • How are you able to view the counter instruction’s current count value as the program runs? • Is there any way to “fool” this system so that it does not hold an accurate count of cars inside the garage?

PLC comparison: • Allen-Bradley Logix 5000: CTUD count-up/down instruction • Allen-Bradley SLC 500: CTU and CTD instructions. • Siemens S7-200: CTUD count-up/down instruction • Koyo (Automation Direct) DirectLogic: UDC counter instruction file i03684 77

Question 58 Question 59 Question 60 Question 61 Suppose we have an Allen-Bradley model “SLC 500” PLC connected to a pair of momentary-contact pushbutton switches and light bulbs as shown in this illustration:

Power supply

Slot 0

Slot 1

Slot 2

Slot 3

(processor)

(discrete input)

(unused)

(discrete output)

Processor

Input 0 1 2 3

L1

120 VAC power

L2/N Gnd

Output 4 5 6 7

0 1 2 3

IN0

VAC 1

IN1

OUT0

IN2

OUT1

IN3

OUT2

IN4

OUT3

IN5

VAC 2

IN6

OUT4

IN7

OUT5

COM

OUT6

COM

OUT7

4 5 6 7

Switch A

Lamp Y

Switch B

Lamp Z

Examine the following relay ladder logic (RLL) program for this Allen-Bradley PLC, determining the statuses of the two lamps provided switch A is pressed by a human operator and switch B is unpressed:

I:1

I:1

O:3

2

6

0

I:1

I:1

O:3

2

6

4

file i03760

78

Question 62 Suppose we have a PLC connected to two pushbutton switches (momentary NO) as shown in this illustration:

120 VAC "line" power L1

X1

L2

L1

L2

X2

Y2

X3 (NO contacts)

X4

Y3 PLC

X5 (NO contacts)

Personal computer display (Ladder Diagram program)

X3

Y4 Y5

X6 Common

Y1

Y6 Programming port

Source

X3

Y1

X5

Y2

X5

X3

Based on the highlighting you see in the “live” PLC program display, determine the status of each pushbutton switch: whether or not each one is being pressed. file i01879

79

Question 63 Read selected portions of the Siemens “SIMATIC S7-200 Programmable Controller System Manual” (document A5E00307987-04, August 2008) and answer the following questions: Identify the different types of SIMATIC timer instructions, explaining how each one functions. Identify a practical application for a timer instruction programmed into a PLC. How long can one of these timer instructions time up to? Based on this maximum value, how many bits do you think are used in the register to store a timer instruction’s current value? What is meant by the resolution of a timer instruction? How many different options do the SIMATIC instructions provide for resolution? Comment on how a SIMATIC timer’s value is updated in a PLC program if the resolution is 1 ms, if it is 10 ms, and if it is 100 ms. The Siemens S7-200 PLC handles each one differently! Sketch a simple ladder-diagram program for a Siemens S7-200 PLC whereby a switch connected to input I0.3 causes a timer to increment (count up) and then turn on an alarm light output Q0.9 after 5 seconds. Suggestions for Socratic discussion • If you have access to your own PLC for experimentation, I urge you to write a simple demonstration program in your PLC allowing you to explore the behavior of these PLC instructions. The program doesn’t have to do anything useful, but merely demonstrate what each instruction does. First, read the appropriate section in your PLC’s manual or instruction reference to identify the proper syntax for that instruction (e.g. which types of data it uses, what address ranges are appropriate), then write the simplest program you can think of to demonstrate that function in isolation. Download this program to your PLC, then run it and observe how it functions “live” by noting the color highlighting in your editing program’s display and/or the numerical values manipulated by each instruction. After “playing” with your demonstration program and observing its behavior, write comments for each rung of your program explaining in your own words what each instruction does. file i00229

80

Question 64 Read selected portions of the Allen-Bradley “MicroLogix 1000 Programmable Controllers (Bulletin 1761 Controllers)” user manual (document 1761-6.3, July 1998) and answer the following questions: Identify the different types of timer instructions available in the MicroLogix 1000 controller, explaining how each one functions. How do these types compare with those offered in the Siemens S7-200 PLC? Identify a practical application for a retentive timer instruction programmed into a PLC. How many different options do the Allen-Bradley MicroLogix timer instructions provide for resolution? How long can one of these timer instructions time up to? Based on this maximum value, how many bits do you think are used in the register to store a timer instruction’s current value? Sketch a simple ladder-diagram program for an Allen-Bradley MicroLogix 1000 PLC whereby a switch connected to input I:0/2 causes a timer to increment (count up) and then turn on an alarm light output O:0/1 after 5 seconds. Suggestions for Socratic discussion • If you have access to your own PLC for experimentation, I urge you to write a simple demonstration program in your PLC allowing you to explore the behavior of these PLC instructions. The program doesn’t have to do anything useful, but merely demonstrate what each instruction does. First, read the appropriate section in your PLC’s manual or instruction reference to identify the proper syntax for that instruction (e.g. which types of data it uses, what address ranges are appropriate), then write the simplest program you can think of to demonstrate that function in isolation. Download this program to your PLC, then run it and observe how it functions “live” by noting the color highlighting in your editing program’s display and/or the numerical values manipulated by each instruction. After “playing” with your demonstration program and observing its behavior, write comments for each rung of your program explaining in your own words what each instruction does. file i00247

81

Question 65 Suppose we have an Allen-Bradley model “SLC 500” PLC connected to a liquid level switch, a selector switch, and a motor contactor (for a pump) as shown in this illustration: Slot 0

Slot 1

Slot 2

(processor) (discrete output)

Power supply

Processor

L2/N Gnd

(discrete input)

Output 0 1 2 3

L1

120 VAC power

Slot 3

(unused)

Input 4 5 6 7

0 1 2 3

VAC 1

IN0

OUT0

IN1

OUT1

IN2

OUT2

IN3

OUT3

IN4

VAC 2

IN5

OUT4

IN6

OUT5

IN7

OUT6

COM

OUT7

COM

4 5 6 7

5 feet

Contactor Left

Right

Explain what conditions must be met for the pump to turn on, based on an analysis of the program running in the PLC:

I:3/4

TON Timer On Delay Timer

I:3/2

EN

T4:3

Time Base

0.1

Preset

250

Accum

0

T4:3/DN

file i04634

82

O:1/0

DN

Question 66 A gravel-crushing operation uses three long conveyor belts to move rock from the quarry to the crusher. The belts must be started up in a particular sequence to avoid overloading the electric motors driving them:

Conveyor A

Conveyor B

Conveyor C

Rock from quarry Rock to crusher M

M

M

PLC Start pushbutton

Stop pushbutton

First, determine a start-up sequence that makes sense: which conveyor belt should start first, next, and last? What might happen if the sequence were reversed? Why not simply start all conveyor motors simultaneously?

83

This operation uses a Siemens S7 series PLC to control the three conveyor belts. Analyze this program and explain how it accomplishes the task of starting up the three conveyors in sequence:

Start I0.1

Stop I0.0

Run M0.0

Run M0.0

Run M0.0

T2

+85

IN

TON

PT

100 ms T3

T2

+85

IN

TON

PT

100 ms

Run M0.0

Conv_C_motor Q0.2

T2

Conv_B_motor Q0.1

T3

Conv_A_motor Q0.0

Lastly, determine where you might add a contact instruction for an emergency shutoff safety switch, so that all three conveyors stop simultaneously if ever the safety switch is actuated. Suggestions for Socratic discussion • How long is the time delay between conveyor start-ups? How might this time delay be altered if needed? • Suppose a warning siren were added to the system, sounding for a full 15 seconds before the first conveyor belt starts. How would you modify the PLC program to include this additional functionality? file i04428

84

Question 67 Suppose we have an Allen-Bradley model “SLC 500” PLC connected to switches and a pump contactor as shown in this illustration: Slot 0

Slot 1

Slot 2

(processor) (discrete output)

Power supply

Processor

120 VAC power

Output

L2/N Gnd

Slot 3 (discrete input)

Input

0 1 2 3

L1

(unused)

4 5 6 7

0 1 2 3

VAC 1

IN0

OUT0

IN1

OUT1

IN2

OUT2

IN3

OUT3

IN4

VAC 2

IN5

OUT4

IN6

OUT5

IN7

OUT6

COM

OUT7

COM

4 5 6 7

3 feet

Contactor

Left

Right

20 PSI

Identify the necessary conditions for the pump to turn on, based on this program running in the PLC: I:3

1

TON Timer On Delay Timer Time Base

I:3

2 I:3

EN

T4:3

Preset

17

Accum

0

T4:3

DN

1.0

O:1

DN

0

I:3

T4:3 RES

0

4

file i04635

85

Question 68 An Allen-Bradley Logix5000 PLC is used to control the starting and stopping of an air compressor based on momentary-contact pushbutton switch inputs as well as high and low pressure switches (PSH and PSL, respectively). Analyze this program and explain how it is supposed to work:

in_start_switch

in_stop_switch

run_enable

run_enable

in_psl

in_psh

run_enable

out_comp_motor

out_comp_motor

out_comp_motor

RTO Retentive Timer On Timer Time Base Preset Accum (continued on next page)

86

run_time 0.001 3600000 0

EN DN

(continued from previous page) run_time.dn

CTU Count Up Counter

hours.dn

CU hours

Preset

250

Accum

0

DN

out_warning_light

in_reset_switch

hours RES

run_time.dn

run_time RES

in_reset_switch

In particular, answer these following questions: • Determine the “normal” electrical statuses of all switches (e.g. NO or NC) connected to the inputs of this PLC, based on an examination of the respective contact instructions within the PLC program. • Why is is important that a retentive timer instruction be used for the calculation of total run-time? • What is the significance of the maintenance warning light controlled by this PLC?

Suggestions for Socratic discussion • Note how all instructions in this Logix5000 PLC program are addressed by tagname rather than by hardware addresses (e.g. I:2/6, O:3/1). How do you suppose the PLC “knows” which real I/O points to associate with which instructions in the program? file i02346

87

Question 69 A technician is troubleshooting a problem with a newly-installed PLC and variable-speed motor drive. One of the discrete (on/off) outputs of the PLC is connected to a discrete input on the drive, to tell the motor to either turn on or turn off. The PLC’s discrete output is a dry contact type, meaning it is nothing more than an electromechanical relay contact inside the output card. The discrete inputs on the drive (DI0, DI1, and DI2) are logic gate inputs, internally “pulled up” with resistors so that the only thing needed to activate each input is to form a connection between the respective input and the common (“Com”) terminal on the drive. The dry contact for PLC output 0 on the right-most output card is supposed to do just that, telling the drive when to start the motor:

Power supply

Processor

Digital inputs

Digital outputs

Digital outputs

0

0

1

1

2

2

3

3

3-phase AC power lines

L1 L2/N

Com

Com

Gnd

L1 L2 L3 DI0 DI1 DI2 Com

T1 T2 T3

To 3-phase motor

The problem is, the motor does not start when the PLC tells it to. Now, the motor itself is brand-new, and the wiring between the motor and the drive is known to be good. A power check at the PLC and drive power terminals shows 117 volts AC between L1 and L2/N (on the PLC) and 482 volts between each of the three phases (L1, L2, and L3) on the motor drive. The LED indicator for output 0 on the PLC card is lit, 88

revealing that the PLC program at least is trying to activate the motor drive. This data suggests (but does not guarantee) that the problem lies either with the PLC hardware or the drive, and not with the power sources, motor wiring, motor, PLC inputs, or PLC program. Both the PLC and the motor drive are complex, programmable devices. The technician knows she could spend quite a bit of time diagnosing either of these devices trying to find a problem. Thus, it would be very helpful to know which of these devices is at fault so as to not waste troubleshooting time. Devise a simple test for the technician to perform that will neatly divide the problem in half, telling her whether the PLC or the drive is at fault, and be sure to explain your reasoning. Suggestions for Socratic discussion • Is the PLC output card sourcing curren to or sinking current from the VFD in this system? • If the problem lies within the PLC, where exactly do you think it might be found within the PLC? Do you think it could be a hardware problem, a software problem, or either? file i02451

89

Question 70 Programming Challenge and Comparison – solenoid valve control with stuck valve alarm A PLC is used to control the opening and closing of a solenoid-operated valve with a single discrete output. A pair of normally-open limit switches sense the valve’s stem position:

PLC Compressed air supply

Processor

Input 0 1 2 3

Valve actuator

Input 4 5 6 7

0 1 2 3

Output 4 5 6 7

0 1 2 3

IN0

IN0

VDC

IN1

IN1

OUT0

IN2

IN2

OUT1

IN3

IN3

OUT2

IN4

IN4

OUT3

IN5

OUT4

IN5

IN6

OUT5

IN7

IN7

OUT6

COM

IN6

COM

OUT7

COM

COM

COM

Full-open limit

4 5 6 7

Alarm lamp

Full-closed limit

Toggle switch Pipe

Pipe Valve

Work individually or in teams to write a PLC program energizing an alarm lamp if the valve fails to reach the full-open position within 5 seconds of receiving the “open” command signal, and energizing the same alarm lamp if the valve fails to reach the full-closed position within 8 seconds of receiving the “close” command signal. Note that the status of both limit switches will be “open” (off) when the stem is between its full-open and full-closed positions. The PLC receives the command to open or close the valve from a hand-operated toggle switch. Inputs • Open/Close toggle – off when commanding valve to shut ; on when commanding valve to open wide • Valve closed limit (NO) – closes when valve reaches 0% position • Valve open limit (NO) – closes when valve reaches 100% position Outputs • Valve actuator solenoid – energizing this coil opens up the valve • “Valve stuck” alarm lamp – energize if valve does not respond in time When your program is complete and tested, capture a screen-shot of it as it appears on your computer, and prepare to present your program solution to the class in a review session for everyone to see and critique. The purpose of this review session is to see multiple solutions to one problem, explore different programming techniques, and gain experience interpreting PLC programs others have written. When presenting your program (either individually or as a team), prepare to discuss the following points: • Identify the “tag names” or “nicknames” used within your program to label I/O and other bits in memory • Follow the sequence of operation in your program, simulating the system in action • Identify any special or otherwise non-standard instructions used in your program, and explain why you decided to take that approach • Show the comments placed in your program, to help explain how and why it works • How you designed the program (i.e. what steps you took to go from a concept to a working program) file i04657

90

Question 71 Programming Challenge – Reversing motor restart delay A three-phase electric motor drives an air heat exchanger (radiative cooler) in either direction (forward or reverse), depending on which way operations personnel wish to blow the warm air. During warm weather, the preferred direction is up, to direct hot air away from process equipment. During cold weather, the preferred direction is down, to provide warmth in the process equipment area to help guard against liquid-filled pipes and tubes freezing:

Radiator core

Forward blows air up

Fan blade

Reverse blows air down

Fan blade 3-phase 480 VAC

Motor

Fwd

Rvs

Reset

A reversing start/stop PLC program is easy enough to write, with two momentary-contact “Start” pushbuttons (one for Forward, one for Reverse) and one momentary “Stop” pushbutton; but what we need here is a reversing program that prevents an immediate re-start of the motor in the opposite direction following a stop command. This is because the fan blades have a lot of inertia, and take about 30 seconds to coast to a stop. This restart lockout timer will prevent someone from trying to reverse the motor’s direction before the fan has had a chance to fully stop. Write a PLC program to provide this forward/reverse/restart lockout functionality. Assume the use of normally-open (NO) pushbutton switches for all pushbutton inputs. Suggestions for Socratic discussion • What type of timer instruction is best suited for this application, an on-delay or an off-delay timer?

file i02347 91

Question 72 Programming Challenge – Reaction time measurement Program your PLC to measure a person’s reaction time in flipping a switch. The PLC should energize a light (or simply one of the discrete output indicating LEDs) telling the user when to flip an input switch, and then the PLC will measure how long it takes for the person to react to the light and flip the switch. Suggestions for Socratic discussion • How can you program the PLC to turn on the signaling light in a way that the person being tested cannot anticipate it? • How must you configure the reaction time timer to count in units appropriate for this very quick time delay? • What type of timer instruction is best suited for the reaction time timer, a retentive or a non-retentive timer?

file i02266 92

Question 73 Programming Challenge – Alarm event latch and history timers A normally-closed (NC) high-pressure sensing switch monitors fluid pressure in a chemical reactor vessel, opening its contacts if the pressure exceeds the trip point. This triggers an alarm lamp to energize in the control room, and this lamp will latch in the “on” state until an operator resets it, even if the high-pressure condition “clears” and goes back to normal. This is so the operators will know a high-pressure event occurred even if they were not in the control room to see it when it happened. A PLC implements this latching function using retentive (“set” and “reset”) coils:

Alarm_output

Pressure_switch_high

S Reset_switch

Alarm_output R

The system works well, but the operators want more. If they arrive at the control room to see the alarm light on (latched), they want to know how long the high-pressure condition lasted and also how long it’s been since the reactor pressure returned to normal. Add instructions to this PLC program to provide the desired timing functionality. Suggestions for Socratic discussion • Explain why the PLC program contact for the high-pressure switch is normally-closed, and how this information alone would be enough for us to determine that the high-pressure switch itself had NC contacts. • What type of timer instruction is best suited for the event duration timer, a retentive or a non-retentive timer? • How could a counter instruction be added to this PLC program to provide useful functionality?

file i02265 93

Question 74 Question 75 Question 76 Question 77 Question 78 Question 79 Question 80

94

Question 81 Suppose we have an Allen-Bradley MicroLogix 1000 PLC connected to a temperature switch and a flow switch:

Trip = 15 GPM

24V

DC COM

I/0

I/1

I/2

I/3

DC COM

I/4

I/5

DC OUT

Power

Trip = 200 oF

Run Fault Force

85-264 VAC

L1

L2/N

VAC VDC

O/0

VAC VDC

O/1

VAC VDC

O/2

VAC VDC

O/3

120 VAC

We wish for the lamp to come on when the temperature is below 200 degrees F and when the flow rate is below 15 GPM. Write a RLL program for the PLC (complete with correct address labels for each of the virtual contacts) to fulfill this function:

O:0

0

file i02375 95

Question 82 Analyze this Allen-Bradley PLC program and explain what it is supposed to do:

Motor O:1

CTU Count Up

0

Counter

CU

C5:2

Preset

17

Accum

0

Start I:0

Stop I:0

C5:2

0

1

DN

DN

Motor O:1 0

Motor O:1 0 Reset I:0

C5:2 RES

2

file i02377 96

Question 83 In relay ladder logic (RLL) programming, it is considered bad practice to have multiple instances of an identical (standard) “relay” coil in a program:

Timer_01

Level_low

Pump_run

Switch_hand

OL_contact

Sump_wet

...

...

Identical coils!

Pump_run

Explain why this is considered poor practice in PLC programming. Next, determine the status of the Pump run output channel given the following bit states: • • • • •

Timer 01 = 1 Level low = 1 Switch hand = 0 OL contact = 0 Sump wet = 0

file i02376 97

Question 84 Sketch the wires necessary to connect two pressure switches and two relay coils to the following AllenBradley MicroLogix 1000 PLC (model 1761-L10BWA, with 6 discrete DC inputs either sourcing or sinking, and 4 discrete relay contact outputs). Be sure to wire the two switches so they source current to the PLC’s inputs (the low-pressure switch to I/2 and the high-pressure switch to I/5, normally-open contacts on both) and wire the relay coils so the PLC sources current to them (O/0 and O/1):

Com

NC

24V

NO

DC COM

I/0

I/1

I/2

I/3

DC COM

I/4

I/5

DC OUT

Power

PSH

Run Fault Force Com

NC

NO

85-264 VAC

L1

L2/N

VAC VDC

O/0

VAC VDC

O/1

VAC VDC

PSL

file i02379 98

O/2

VAC VDC

O/3

Question 85 Analyze this Siemens S7-200 PLC program (for controlling a motor) and explain what it is supposed to do:

T6

M0.2

+50 Start I0.4

IN

TON

PT

100 ms

Stop I0.5

M0.2

M0.2

T6

Low speed Q0.0

M0.2

T6

High speed Q0.1

M0.2

Include an explanation of the motor contactor wiring, based on an analysis of the PLC program.

file i02255 99

Question 86 Calculate all voltages, currents, and total power in this balanced Y-Y system:

Source

Load

7 27

580 Ω

V

• Eline = • Iline = • Ephase(source) = • Iphase(source) = • Ephase(load) = • Iphase(load) = • Ptotal =

file i02421 100

Question 87 The following circuit senses temperature using a thermistor with a positive temperature coefficient (i.e. resistance increases as temperature increases):

G

D

A

10 VDC 1k

Cable 1

(80 mA current-limited)

Cable 2

B

E

H

C

F

J

+ −

Voltmeter

+To

1k (at room temp)

First, determine the voltage we should read at the voltmeter with the thermistor at or near room temperature. Next, identify the likelihood of each specified fault for this circuit, supposing the voltmeter registers 0 volts with the thermistor at room temperature, and a voltage measurement taken between terminals D and F registers 10 volts. Consider each fault one at a time (i.e. no multiple faults), determining whether or not each fault could independently account for all measurements and symptoms in this circuit. Fault Thermistor failed open Fixed resistor failed open Wire A-D failed open Wire F-J failed open Wire E-H failed open Thermistor failed shorted Fixed resistor failed shorted Short between terminals G-H Short between terminals E-F Short between terminals D-E

file i02924 101

Possible

Impossible

Question 88 Identify each of the specified voltages in the following circuit, with reference to ground. The subscripts refer to the specific test points (where the red test lead of the voltmeter touches the circuit):

A

B

3 kΩ C

18 V

5 kΩ

E

D 1 kΩ

For example, VB means the voltage indicated by a voltmeter with the red test lead touching point B and the black test lead touching ground. • VA = • VB = • VC = • VD = • VE =

file i02525 102

Question 89 Suppose a single-phase AC load draws a current of 16.5 amps at 237 volts (RMS). If the measured power factor of this load is 0.85, calculate the true power (P ) dissipated by the load as well as its apparent power (S). Be sure to include the proper unit of measurement (e.g. VA, VAR, or W) with each answer! P =

S=

file i02422 103

Question 90 In this 480 volt AC induction motor control circuit (sometimes referred to as a “bucket”), a three-pole relay (typically called a contactor) is used to switch power on and off to the motor. The contactor itself is controlled by a smaller switch, which receives 120 volts AC from a step-down transformer to energize the contactor’s magnetic coil. Although this motor control circuit used to work just fine, today the motor refuses to start.

To 3-φ , 480 volt power source

L1

L2

L3

L1

L2

L3

Schematic diagram Fuses

Transformer X2

H1

H3

H2

H4

X1

Contactor

H1

H3

H2

H4

Transformer

Contactor A1

X1 A2

A2

motor

Switch

T1 T2

Motor

X2

A1

T3

Switch

T1 T2 T3

Using your AC voltmeter, you measure 476 volts AC between L1 and L2, 477 volts AC between L2 and L3, and 475 volts AC between L1 and L3. You also measure 477 volts between transformer terminals H1 and H4. With the switch in the “on” position, you measure 0.5 volts AC between terminals X1 and X2 on the transformer. From this information, identify the following: • Two components or wires in the circuit that you know cannot be failed either open or shorted, besides the 480 volt AC source which is obviously operational.

• Two different component or wire failures in the circuit, either one of which could account for the problem and all measured values, and the types of failures they would be (either open or shorted).

file i03174 104

Question 91 Lab Exercise – introduction Your team’s task is to construct a system controlled by a PLC. The system you choose to build shall use (at minimum) discrete input(s), discrete output(s), and either counter or timer functions. This system will be expanded during the next course to include a three-pole contactor, so designing the system with this in mind (or simply installing the contactor in this exercise) will save you time later. Project ideas include: • Air compressor control, with high and low air pressure switches • Water sump pump control, with high and low water level switches • Other alternatives? Must be pre-approved by instructor! In addition to functioning properly, the PLC program must be fully documented and edited for cleanliness and good programming form. This includes labels (aliases, or symbolic names) for all inputs and outputs, and comments for each and every rung of logic explaining the rungs’ functions. Although there will be only one program submitted by each team, completion of this objective is individual, with each student explaining (at least) a part of the PLC program to the instructor. Objective completion table: Performance objective Prototype sketch (before building the system!) Complete I/O list Prototype PLC program (before programming!) Final wiring diagram and system inspection Demonstration of working system Final PLC program inspection Lab question: Selection/testing Lab question: Commissioning Lab question: Mental math Lab question: Diagnostics

Grading mastery mastery mastery mastery mastery mastery proportional proportional proportional proportional

1 – – –

2 – – –

3 – – –

4 – – –

–

–

–

–

Team

–––– – – – – –

– – – – –

– – – – –

– – – – –

The only “proportional” scoring in this activity are the lab questions, which are answered by each student individually in a private session between the instructor and the team. A listing of potential lab questions are shown at the end of this worksheet question. The lab questions are intended to guide your labwork as much as they are intended to measure your comprehension, and as such the instructor may ask these questions of your team day by day, rather than all at once (on a single day). It is essential that your team plans ahead what to accomplish each day. A short (10 minute) team meeting at the beginning of each lab session is a good way to do this, reviewing what’s already been done, what’s left to do, and what assessments you should be ready for. There is a lot of work involved with building, documenting, and troubleshooting these working instrument systems! As you and your team work on this system, you will invariably encounter problems. You should always attempt to solve these problems as a team before requesting instructor assistance. If you still require instructor assistance, write your team’s color on the lab whiteboard with a brief description of what you need help on. The instructor will meet with each team in order they appear on the whiteboard to address these problems.

105

Lab Exercise – planning the wiring One of the most common problems students encounter when building any working system, whether it be a circuit on a solderless breadboard or an instrument loop spanning an entire room, is properly connecting and configuring all components. An unfortunate tendency among most students is to simply start connecting parts together, essentially designing the system as they go. This usually leads to improperly-connected components and non-functioning systems, sometimes with the result of destroying components due to those improper connections! An alternative approach is to plan ahead by designing the system before constructing it. This is easily done by sketching a diagram showing how all the components should interconnect, then analyzing that diagram and making changes before connecting anything together. When done as a team, this step ensures everyone is aware of how the system should work, and how it should go together. The resulting “prototype” diagram need not be complex in detail, but it should be detailed enough for anyone to see which component terminals (and ports) connect to terminals and ports of other devices in the system. For example, your team’s prototype sketch should be clear enough to determine all DC electrical components will have the correct polarities. If your proposed system contains a significant amount of plumbing (pipes and tubes), your prototype sketch should show all those connections as well. Your first step should be selecting a PLC to use for this project. The PLC needs to be one with modular (add-on) I/O cards to provide sufficient complexity for the project. Monolithic “brick” PLCs (with no add-on I/O modules) are not acceptable for this project. An Allen-Bradley SLC 500 PLC would be a good choice, as well as a Siemens S7 series or an AutomationDirect Productivity 3000. You will also need to select appropriate field devices (switches, pumps, etc.) for your project. You are free to use the field devices left over from the relay-based motor control lab if you prefer. The next step should be finding appropriate documentation for your PLC. All PLC manufacturers provide manuals and datasheets for their products online. Use this documentation to identify how to properly wire, power, and program your team’s PLC. PLC equipment manuals always provide sample diagrams showing how external components may connect to the I/O points. Feel free to use these sample diagrams as templates for your prototype sketch. This is the most challenging portion of your wiring, so be sure to work with your teammates to get this right! Your team’s prototype sketch is so important that the instructor will demand you provide this plan before any construction on your team’s working system begins. Any team found constructing their system without a verified plan will be ordered to cease construction and not resume until a prototype plan has been drafted and approved! Each member on the team should have ready access to this plan (ideally possessing their own copy of the plan) throughout the construction process. Prototype design sketching is a skill and a habit you should cultivate in school and take with you in your new career. Planning a functioning system should take no more than a couple of hours if the team is working efficiently, and will save you hours of frustration (and possible component destruction!).

106

Lab Exercise – developing an I/O list It is a good idea when programming any computer system to first identify all the input and output signals to the system, as well as internal variables if possible, before commencing on the development of the program itself. In order to reinforce this practice, your team will be required to develop a complete list of all input and output points on your proposed system along with any tagnames (also known as “symbols” or “nicknames”) identifying the function of each. Once this list is complete and you are ready to begin developing the PLC program, you can enter all the tagnames and define the I/O points as your very first programming step. With this data in place, the writing of your program will be made easier because each I/O tag you reference will already be defined and labeled, reminding you of their functions within the system. Here is a sample I/O list for a motor control PLC program: Hardware I/O terminal Card 1, terminal IN0

I/O type 24 VDC discrete input

Tagname START PB

Card 1, terminal IN1

24 VDC discrete input

STOP PB


24 VDC discrete input

E STOP


4-20 mA analog input

MTR TEMP

Card 3, terminal OUT0

120 VAC discrete output

CONTACTOR

107

Notes Black pushbutton, momentary NO contacts Red pushbutton, momentary NC contacts Red pushbutton, latching NC contacts Current signal scaled 0 to 150 deg F To terminal A1

Lab Exercise – wiring the system The Instrumentation lab is set up to facilitate the construction of working systems, with over a dozen junction boxes, pre-pulled signal cables, and “racks” set up with 2-inch vertical pipes for mounting instruments. The only wires you should need to install to build a working system are those connecting the field instrument to the nearest junction box, and then small “jumper” cables connecting different pre-installed cables together within intermediate junction boxes. Your team’s PLC must be installed in a suitable electrical enclosure, with AC power fed to it through a fuse or circuit breaker (on the “hot” conductor only), and firmly grounded (the ground conductor of the power cord securely fastened to the metal frame of the enclosure and the PLC chassis). All I/O wiring should be neatly loomed together and/or run through wire duct (“Panduit”). Power to I/O cards must be routed through their own fuses so that I/O power may be disconnected independently of power to the PLC processor and rack. Common mistakes: • Neglecting to consult the manufacturer’s documentation for field instruments (e.g. how to wire them, how to calibrate them). • Proceeding with wiring before creating an initial sketch of the circuitry and checking that sketch for errors. • Mounting the field instrument(s) in awkward positions, making it difficult to reach connection terminals or to remove covers when installed. • Failing to tug on each and every wire where it terminates to ensure a mechanically sound connection. • Students working on portions of the system in isolation, not sharing with their teammates what they did and how. It is important that the whole team learns all aspects of their system! Building a functioning system should take no more than one full lab session (3 hours) if all components are readily available and the team is working efficiently!

108

Lab Exercise – programming the system Like wiring a control system, programming one is best done with thoughtful planning rather than a “design-as-you-build” approach. Each team will work with the instructor to develop a “prototype” PLC program, usually on a whiteboard or on paper. Your prototype program should completely address the following points: • • • • •

Identify Identify Identify Identify Identify

all all all all all

inputs to the PLC, giving each one a sensible tagname signal outputs from the PLC, giving each one a sensible tagname major program functions (i.e. What must this program do?) internal variables necessary for these functions, giving each one a sensible tagname system variables necessary for these functions (e.g. real-time clock/calendar variables)

The importance of identifying and naming all relevant variables is paramount to “clean” programming. This is especially true when an HMI (Human-Machine Interface) is to be connected to the PLC, and all relevant variables must be named there as well. A reasonable approach to developing a robust program prototype is to create your prototype in your own personal (“brick”) PLC, de-bugging it there with all the switches in place to simulate input signals. Even if your personal PLC is a different model (or manufacture) than the project PLC, this is a very helpful exercise. Furthermore, it allows you to continue program development outside of school when you do not have access to the project PLC. Only after a prototype program is developed should you begin programming the project PLC. I recommend the following steps: • • • • • • • •

Establish communications between PLC and personal computer (PC) Connect all I/O cards (modules) to the PLC and get them recognized by the processor Assign tagnames to all relevant variables, beginning with I/O points Enter a simplified version of the program, running to check for “bugs” Diagnose any program problems Add complexity to the program (e.g. additional features) and run to check for “bugs” Repeat last two steps as often as necessary Add comments to each and every line of the program, explaining how it functions

The final program should be well-documented, clean, and as simple as possible. All members of the team should have a hand in designing the program, and everyone must thoroughly understand how it works. Common mistakes: • Waiting too long after writing the program code to insert comments. This is best done immediately, while everything makes sense and is fresh in your memory! • Insufficient commenting – only makes sense to the person who did the programming • Students working on portions of the program in isolation, not sharing with their teammates what they did and how. It is important that the whole team learns all aspects of their system!

109

Lab Exercise – documenting the system Each student must sketch their own wiring diagram for their team’s system, following industry-standard conventions. Sample diagrams for input and output wiring are shown in the next question in this worksheet. These wiring diagrams must be comprehensive and detailed, showing every connection, every cable, every terminal block, etc. The principle to keep in mind here is to make the wiring diagram so complete and unambiguous that anyone can follow it to see what connects to what, even someone unfamiliar with industrial instrumentation. In industry, control systems are often constructed by contract personnel with limited understanding of how the system is supposed to function. The associated diagrams they follow must be so complete that they will be able to connect everything properly without necessarily understanding how it is supposed to work. When your entire team is finished drafting your individual wiring diagrams, call the instructor to do an inspection of the system. Here, the instructor will have students take turns going through the entire system, with the other students checking their diagrams for errors and omissions along the way. During this time the instructor will also inspect the quality of the installation, identifying problems such as frayed wires, improperly crimped terminals, poor cable routing, missing labels, lack of wire duct covers, etc. The team must correct all identified errors in order to receive credit for their system. After successfully passing the inspection, each team member needs to place their wiring diagram in the diagram holder located in the middle of the lab behind the main control panel. When it comes time to troubleshoot another team’s system, this is where you will go to find a wiring diagram for that system! Common mistakes: • • • • •

Forgetting to label all signal wires (see example wiring diagrams). Forgetting to label all field instruments with their own tag names (e.g. PSL-83). Forgetting to note all wire colors. Forgetting to put your name on the wiring diagram! Basing your diagram off of a team-mate’s diagram, rather than closely inspecting the system for yourself.

Creating and inspecting accurate wiring diagrams should take no more than one full lab session (3 hours) if the team is working efficiently!

110

Lab questions – (reviewed between instructor and student team in a private session) • • • • • • •

Selection and Initial Testing Explain what is meant by the term “sinking” with regard to a PLC input card (DC) Explain what is meant by the term “sourcing” with regard to a PLC input card (DC) Explain what is meant by the term “sinking” with regard to a PLC output card (DC) Explain what is meant by the term “sourcing” with regard to a PLC output card (DC) Explain what a “TRIAC” PLC output card is, and how it differs from DC output cards Explain what a “relay” PLC output card is, and how it differs from sourcing or sinking DC output cards

• Commissioning and Documentation • Demonstrate how to isolate potentially hazardous energy in your system (lock-out, tag-out) and also how to safely verify the energy has been isolated prior to commencing work on the system • Demonstrate how to insert text comments for a rung in a PLC program • Demonstrate how to insert text comments for a single bit (contact or coil) in a PLC program • Demonstrate how to switch between online and offline modes in the PLC programming software • Demonstrate how to transfer a program from the PLC to the programming computer (PC) • Demonstrate how to transfer a program from the programming computer (PC) to the PLC • • • • • •

Mental math (no calculator allowed!) Convert a binary number into decimal Convert a binary number into hexadecimal Convert a decimal number into binary Convert a hexadecimal number into binary Convert a hexadecimal number into decimal

• Diagnostics • “Virtual Troubleshooting” – referencing their system’s diagram(s), students propose diagnostic tests (e.g. ask the instructor what a meter would measure when connected between specified points; ask the instructor how the system responds if test points are jumpered) while the instructor replies according to how the system would behave if it were faulted. Students try to determine the nature and location of the fault based on the results of their own diagnostic tests. • Demonstrate how to verify a sinking discrete input’s status using a voltmeter • Demonstrate how to verify a sourcing discrete input’s status using a voltmeter • Demonstrate how to verify a sinking discrete output’s status using a voltmeter • Demonstrate how to verify a sourcing discrete output’s status using a voltmeter • Describe how to replace a failed I/O card in a PLC, assuming that card is hot-swappable • Describe how to replace a failed I/O card in a PLC, assuming that card is not hot-swappable • Identify status of a discrete input field device (e.g. switch) by examining the status of its corresponding contact instruction in the PLC program (colored versus uncolored)

111

Wiring diagram requirements • Wiring diagram • Proper symbols and designations used for all components. • Relay coil and contacts properly named. • Text descriptions • Each instrument documented below (tag number, description, etc.). • Calibration (input and output ranges) given for each instrument, as applicable. • • • • • • •

Connection points All terminal blocks properly labeled. All terminals shown in proper order on diagram. All I/O cards and points fully labeled (complete with program addresses). All wires are numbered. All electrically-common points in the circuit shall bear the same wire number. All wire colors shown next to each terminal.

• Cables and tubes • Multi-pair cables or pneumatic tube bundles going between junction boxes and/or panels need to have unique numbers (e.g. “Cable 10”) as well as numbers for each pair (e.g. “Pair 1,” “Pair 2,” etc.). • Energy sources • All power source intensities labeled (e.g. “24 VDC,” “120 VAC,” “20 PSI”) • All shutoff points labeled (e.g. “Breaker #5,” “Valve #7”) file i03655 Question 92 Wiring diagram requirements Perhaps the most important rule to follow when drafting a wiring diagram is your diagram should be complete and detailed enough that even someone who is not a technician could understand where every wire should connect in the system! • Field device symbols • Proper electrical symbols and designations used for all field devices. • Optional: Trip settings written next to each process switch. • PLC I/O cards • All terminals labeled, even if unused in your system. • Model number, I/O type, and PLC slot number should be shown for each and every card. • • • • • • •

Connection points All terminals properly labeled. All terminal blocks properly labeled. All junction (“field”) boxes shown as distinct sections of the loop diagram, and properly labeled. All control panels shown as distinct sections of the loop diagram, and properly labeled. All wire colors shown next to each terminal. All terminals on devices labeled as they appear on the device (so that anyone reading the diagram will know which device terminal each wire goes to).

• Energy sources • All power source intensities labeled (e.g. “24 VDC,” “120 VAC,” “480 VAC 3-phase”) • All shutoff points labeled (e.g. “Breaker #5,” “Valve #7”)

112

PB-5 Reset

Trips @ 35 PSI rising

PSH-10

Field

C

NO

COM

NC

Blk

Red

Blk

Red

Cable PB-5

Cable PSH-10

Blk

Red

Blk

Red

12

11

2

1

TB-43 Grn

Gry

Org

Field junction box JB-28

Blu

Blu

Blu

Blu

Blu

Blu

Blu

Blu

113

Gry

4

3

2

1

Red

Red

24VDC Power supply

COM

L2

L1

Slot 4

120 VAC Bkr #3

24VDC sinking

1746-IB8 Discrete input

Red

Blk

COM

IN7

IN6

IN5

IN4

IN3

IN2

IN1

IN0

Red

(1 amp each)

Fuse block

8

7

6

5

4

3

2

1

TB-7

PLC cabinet

Sample Input Wiring Diagram

PAH-20 Alarm lamp

Trip solenoid PY-3

Field

Blk

Red

Cable PAH-20

Blk

Red

Blk

Blk

8

7

6

5

TB-44 Red

Red

Cable PY-3

Wht

Blk

Blk

Field junction box JB-28

Wht

Blk

Blk

Wht

4

3

2

1

Blu

Blu

Blu

Blu

TB-11

file i01880

114

Blk

3

2

1

Blk

Blk

Blk

(1 amp each)

Fuse block

OUT7

OUT6

OUT5

OUT4

VAC2

OUT3

OUT2

OUT1

OUT0

VAC1

Wht

Blk

120 VAC Bkr #1

Slot 1

100-240 VAC TRIAC

1746-OA8 Discrete output

PLC cabinet

Sample Output Wiring Diagram

Question 93 Connect an “ice cube” relay to one of the outputs on a PLC, so that the PLC can control the energization of the relay. All electrical connections must be made using a terminal strip (no twisted wires, crimp splices, wire nuts, spring clips, or “alligator” clips permitted). Program this PLC to implement a motor start/stop (latching) control function. In order to ensure your program has not been pre-written in your computer prior to this assessment, you will be asked to sketch a correct ladder-diagram PLC program on paper to implement this function prior to using a computer. You must connect a “commutating” diode in parallel with the relay’s coil to prevent the phenomenon known as “inductive kickback,” which may otherwise damage the transistor output on a PLC. Note that incorrectly connecting this diode will present a short-circuit to the PLC, so you must get it right! This exercise tests your ability to properly interpret the “pinout” of an electromechanical relay, properly wire a PLC output channel to control a relay’s coil, properly polarize a commutating diode to prevent inductive kickback from damaging the PLC output, and use a terminal strip to organize all electrical connections. It also tests your ability to program motor start/stop logic using either a seal-in contact or latching (retentive) coil instructions.

PLC 24V

DC COM

I/0

I/1

I/2

I/3

Relay socket

DC COM

I/4

Terminal strip

I/5

DC OUT

Relay


Diode

85-264 VAC

L1

L2/N

VAC VDC

O/0

VAC VDC

O/1

VAC VDC

O/2

VAC VDC

O/3

The following components and materials will be available to you during the exam: assorted “ice cube” relays with DC-rated coils and matching sockets ; terminal strips ; 1N400X rectifying diodes ; lengths of hook-up wire. You will be expected to supply your own screwdrivers and multimeter for assembling and testing the circuit at your desk. “Start” switch to input: PLC program (instructor chooses):

“Stop” switch to input: Seal-in contact

file i00127 115

Relay to output: Retentive coils

Answers Answer 1 Answer 2 Input register, byte 1, bit 4: I1.4 Output register, byte 0, bit 2: Q0.2 Variable memory double word, starting at byte 105: VD105 (a double-word consisting of 4 bytes, or 32 bits) Answer 3 Input file, element 1, bit 4: I:1/4 Output file, element 0, bit 2: O:0/2 Timer 6 accumulator word: T4:6.ACC Answer 4 For the Allen-Bradley MicroLogix example, the lamp will energize only when switch 0 is turned off and switch 1 is turned on. For the Siemens S7-200 example, the lamp will energize when switch 0 is turned on or if switch 1 is turned off, or both conditions occur simultaneously. For the Koyo example, the lamp will energize according to the Exclusive-OR function with switch 1 and switch 2. The lamp energizes when switch 1 is on and switch 2 is off, or when switch 1 is off and switch 2 is on. Answer 5 Answer 6 Answer 7 Answer 8 Answer 9

116

Answer 10 Demonstration program showing some basic bit instructions in an Allen-Bradley MicroLogix PLC:

When the switch connected to input 0 is turned on, the input bit I:0/0 goes from 0 to 1, and this contact becomes colored on my laptop PC’s screen. That color is sent to the coil instruction, where it turns on output bit O:0/1. This makes output channel 1 turn on, energizing the light bulb wired to that output. When I turn off input switch 0, the contact un-colors and so does the output coil O:0/1. This program rung makes output O:0/1 be the same state as input I:0/0.

I:0/0

O:0/1

When the same switch on input 0 is turned on, the input bit I:0/0 goes from 0 to 1, and this contact becomes un-colored. This makes the output bit O:0/2 turn off, so that O:0/2 is always the opposite state of I:0/0.

I:0/0

O:0/2

Placing these two contact instruction in "series" with each other makes it so the coil only gets colored if both of the contacts become colored. O:0/3 turns on only if switch 4 is on and switch 5 is off.

I:0/4

I:0/5

O:0/3

117

Demonstration program showing an “up” counter instruction in an Allen-Bradley MicroLogix PLC:

The CTU instruction is a counter that counts in the "up" direction when its input is toggled. When the "Preset" count value reached, the "Done" bit (DN) activates.

I:0/0

CTU Count Up Counter

CU

C5:0

Preset

12

Accum

5

DN

When the counter C5:0 reaches the "done" condition, the contact C5:0/DN becomes colored, passing color to the coil O:0/0 to turn on a light bulb connected to output 0.

O:0/0

C5:0/DN

Allen-Bradley counter instructions can only be reset by external commands, in this case a special coil instruction sharing the same address as the counter instruction (C5:0). Activating the I:0/1 input causes the RES coil to become colored, which then resets the CTU instruction’s "Accumulated" value back to zero.

C5:0

I:0/1

RES

118

Demonstration program showing an on-delay timer instruction in an Allen-Bradley MicroLogix PLC:

The TON instruction is a timer. Its "Accum" value starts at 0 and counts up (1...2...3...4...) whenever the input contact is colored. When the "Accum" value reaches 5, the DN coil becomes colored. The "Time Base" value of 1.0 means that each count of the "Accum" is 1.0 seconds’ worth of time. If I make the "Time Base" something different, the timer will count faster.

I:0/0

TON Timer On Delay Timer Time Base Preset

EN

T4:0

DN

1.0 5

Accum

When the timer T4:0 reaches the "done" condition, the contact T4:0/DN becomes colored, passing color to the coil O:0/0 to turn on a light bulb connected to output 0.

T4:0/DN

O:0/0

Answer 11 Answer 12 Bit statuses: • I:0/0 = 1 • I:0/1 = 0 • I:0/2 = 1 Answer 13 Bit statuses: • I0.2 = 1 • I1.1 = 0 Answer 14 L > 3 feet, P > 37 PSI, and T < 88o F

119

Answer 15 • • • • •

I0.2 I0.5 I1.1 Q0.1 Q0.6

= = = = =

0 1 0 1 0

Answer 16 Switch statuses: • Switch A = released • Switch B = pressed • Switch C = released The lamp will be energized. Answer 17

All currents shown using conventional flow notation Answer 18

Mtr

120

Answer 19

Circuit 1

This will work!

Circuit 2

Load

Circuit 3

This will work!

Load

This circuit is bad

Circuit 4

This circuit is bad

Load Load

121

Answer 20

Circuit 1

This circuit is bad

Circuit 2

This will work!

Load

Load

Circuit 3

This circuit is bad

Circuit 4

This will work!

Load Load

122

Answer 21 Demonstration program showing some basic bit instructions in an Allen-Bradley MicroLogix PLC:

When the switch connected to input 0 is turned on, the input bit I:0/0 goes from 0 to 1, and this contact becomes colored on my laptop PC’s screen. That color is sent to the coil instruction, where it turns on output bit O:0/1. This makes output channel 1 turn on, energizing the light bulb wired to that output. When I turn off input switch 0, the contact un-colors and so does the output coil O:0/1. This program rung makes output O:0/1 be the same state as input I:0/0.

I:0/0

O:0/1

When the same switch on input 0 is turned on, the input bit I:0/0 goes from 0 to 1, and this contact becomes un-colored. This makes the output bit O:0/2 turn off, so that O:0/2 is always the opposite state of I:0/0.

I:0/0

O:0/2

Placing these two contact instruction in "series" with each other makes it so the coil only gets colored if both of the contacts become colored. O:0/3 turns on only if switch 4 is on and switch 5 is off.

I:0/4

I:0/5

O:0/3

Note: your own demonstration program should contain some retentive coil instruction as well, in order for you to be able to observe what these instructions do and how their operation differs from that of “regular” coil instructions! Answer 22

X1

X2

X2

X1

X3

• Y1 = 1

123

Y1

Answer 23 • • • •

I0.7 I1.1 Q0.1 Q0.3

= = = =

0 1 0 1

Answer 24 Neither output will activate to energize either lamp. Answer 25 Neither output will activate, resulting in both lamps de-energized. Answer 26 Necessary switch statuses: • Switch A = released or Switch B = pressed • Switch C = pressed Answer 27 Some possible problems to account for what we are seeing: • • • •

Overload contact tripped (open) Wire connecting OL contact to I:0/3 failed open Wire connecting “Stop” switch to OL contact failed open Input channel I:0/3 defective on the PLC

Answer 28 Answer 29 Answer 30 Answer 31 Bit statuses: • I:0/0 = 0 • I:0/1 = 0 • I:0/3 = 1 Answer 32 Bit statuses: • I0.2 = 0 • I1.1 = 0 Answer 33 Bit statuses: • I:1/3 = 1 • I:1/5 = 0

124

Answer 34 Answer 35 Remember that a bipolar transistor requires current through the base-emitter junction in order to turn on, and thereby let load current pass between collector and emitter.

Circuit 1 This circuit will work!

Circuit 4

This circuit is bad

Circuit 2

This circuit is bad

Circuit 5 This circuit will work!

Circuit 3

This circuit is bad

Circuit 6

This circuit is bad

Circuit #3 is different from the other “bad” circuits. While the other bad circuits’ lamps do not energize at all, the lamp in circuit #3 energizes weakly when the pushbutton switch is open (not actuated). This is due to the fact that lamp current will naturally pass through the base-collector PN junction as though it were a simple diode, regardless of the switch’s state. Answer 36 Circuits 3, 5, and 6 are flawed, because the emitter-base junctions of their transistors are overpowered every time the switch closes. Hint: draw the respective paths of switch and lamp current for each circuit!

125

Answer 37

Contact points

126

Answer 38

+V

+V

+V

Load NPN Switch sourcing current to transistor




PNP Load

Follow-up question: explain why neither of the following transistor circuits will work. When the pushbutton switch is actuated, the load remains de-energized:

+V

+V

+V

Load

Load

127

Answer 39

+V

+V Switch sinking current from transistor

+V Load

PNP



NPN Load

Switch sourcing current to transistor

Follow-up question: explain why neither of the following transistor circuits will work. When the pushbutton switch is actuated, the load remains de-energized:

+V

+V

+V Load

Load

Answer 40 Answer 41 Answer 42 Switch statuses: • Pressure switch = less than 30 PSI • Temperature switch = cooler than 150 deg F • Level switch = greater than 4 inches The lamp will be de-energized. Answer 43 This PLC program allows the motor to start up 7 times. If you thought the correct number of start-ups was eight, consider the fact that the counter’s output bit (CT1) gets set when the counter’s current value equals the SetPoint value, not when it exceeds the SetPoint value.

128

Answer 44 Answer 45 Answer 46 Answer 47 Hint: the “P” contact instructions are positive transition instructions, “activating” whenever their respective bits transition from 0 to 1, but returning to an “inactive” state whenever the bit value holds at either 0 or 1. Answer 48 Answer 49 Answer 50 This is one possible fix for the problem:

PLC program IN_switch_Start

OUT_valve

IN_switch_Stop

OUT_valve

IN_oil_press

Answer 51 Hint: the contact address C5:0.ACC/13 refers to the 13th bit of the counter’s accumulator register, which is a 16-bit binary number. The 15th bit would be the MSB, while the 0th bit is the LSB. Answer 52 Each “wasteful” program uses an output bit as the intermediary bit between the AND and NOT functions when there is no need. Answer 53 Answer 54 Temperature = below 135 o F Level = above 23 inches Pressure = below 17 PSI

129

Answer 55 If the lamp is energized, we know that the top two virtual contacts (X1 and X2) are colored, and/or the bottom two virtual contacts (X3 and X2) are colored. For the top two virtual contacts to be colored, X1 must be 0 and X2 must be 1. This equates to a pressure less than 32 PSI and a level less than 10 inches. For the bottom two virtual contacts to be colored, X3 must be 1 and X2 must be 0. This equates to a temperature greater than 99 o F and a level greater than 10 inches. Answer 56 Answer 57 Answer 58 Answer 59 Answer 60 Answer 61 Output O:3/4 will activate to energize lamp Y, but the other output (and lamp) will remain off. Answer 62 Answer 63 Answer 64 Answer 65 The liquid level must exceed 5 feet in height for at least 25 seconds and the selector switch must be in the “right” position in order for the pump to turn on. Answer 66 Although starting all three conveyor motors simultaneously would be very simple, it would be a bad thing to do because of the inrush current of all three motors placing undue load on the power system. Answer 67 Necessary conditions to start the pump: • Pressure must be less than 20 PSI for at least 17 seconds • Selector switch must be in the “left” position • Pushbutton must be released (unpressed) or level more than 3 feet Answer 68 Input switch electrical “normal” statuses: • • • • •

Start = NO Stop = NC PSL = NC PSH = NC Reset = NO

130

Answer 69 Here is a schematic diagram to help you formulate an answer:

PLC discrete output

Motor drive discrete input DI0

Program

0

Vdd

Vdd

Com

Com

Answer 70 Answer 71 Answer 72 Answer 73 Answer 74 Answer 75 Answer 76 Answer 77 Answer 78 Answer 79 Answer 80 Answer 81 This is a graded question – no answers or hints given! Answer 82 This is a graded question – no answers or hints given! Answer 83 This is a graded question – no answers or hints given! Answer 84 This is a graded question – no answers or hints given! Answer 85 This is a graded question – no answers or hints given!

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Answer 86 This is a graded question – no answers or hints given! Answer 87 This is a graded question – no answers or hints given! Answer 88 This is a graded question – no answers or hints given! Answer 89 This is a graded question – no answers or hints given! Answer 90 This is a graded question – no answers or hints given! Answer 91 Answer 92 Your loop diagram will be validated when the instructor inspects the loop with you and the rest of your team. Answer 93

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plc programming examples.pdf

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